06.02-Lustre Parameters.md

Lustre Parameters

• Lustre Parameters
  • Introduction to Lustre Parameters
    • Identifying Lustre File Systems and Servers
  • Tuning Multi-Block Allocation (mballoc)
  • Monitoring Lustre File System I/O
    • Monitoring the Client RPC Stream
    • Monitoring Client Activity
    • Monitoring Client Read-Write Extent Statistics
      • Client-Based I/O Extent Size Survey
      • Per-Process Client I/O Statistics
    • Monitoring the OST Block I/O Stream
  • Tuning Lustre File System I/O
    • Tuning the Client I/O RPC Stream
    • Tuning File Readahead and Directory Statahead
      • Tuning File Readahead
      • Tuning Directory Statahead and AGL
    • Tuning OSS Read Cache
      • Using OSS Read Cache
    • Enabling OSS Asynchronous Journal Commit
    • Tuning the Client Metadata RPC Stream
      • Configuring the Client Metadata RPC Stream
      • Monitoring the Client Metadata RPC Stream
  • Configuring Timeouts in a Lustre File System
    • Configuring Adaptive Timeouts
      • Interpreting Adaptive Timeout Information
    • Setting Static Timeouts
  • Monitoring LNet
  • Allocating Free Space on OSTs
  • Configuring Locking
  • Setting MDS and OSS Thread Counts
  • Enabling and Interpreting Debugging Logs
    • Interpreting OST Statistics
    • Interpreting MDT Statistics

The /proc and /sys file systems acts as an interface to internal data structures in the kernel. This chapter describes parameters and tunables that are useful for optimizing and monitoring aspects of a Lustre file system. It includes these sections:

• the section called “Enabling and Interpreting Debugging Logs”

Introduction to Lustre Parameters

Lustre parameters and statistics files provide an interface to internal data structures in the kernel that enables monitoring and tuning of many aspects of Lustre file system and application performance. These data structures include settings and metrics for components such as memory, networking, file systems, and kernel housekeeping routines, which are available throughout the hierarchical file layout.

Typically, metrics are accessed via lctl get_param files and settings are changed by via lctl set_param. While it is possible to access parameters in /proc and /sys directly, the location of these parameters may change between releases, so it is recommended to always use lctl to access the parameters from userspace scripts. Some data is server-only, some data is client-only, and some data is exported from the client to the server and is thus duplicated in both locations.

Note

In the examples in this chapter, # indicates a command is entered as root. Lustre servers are named according to the convention *fsname*-*MDT|OSTnumber*. The standard UNIX wildcard designation (*) is used.

Some examples are shown below:

• To obtain data from a Lustre client:

  # lctl list_param osc.*
  osc.testfs-OST0000-osc-ffff881071d5cc00
  osc.testfs-OST0001-osc-ffff881071d5cc00
  osc.testfs-OST0002-osc-ffff881071d5cc00
  osc.testfs-OST0003-osc-ffff881071d5cc00
  osc.testfs-OST0004-osc-ffff881071d5cc00
  osc.testfs-OST0005-osc-ffff881071d5cc00
  osc.testfs-OST0006-osc-ffff881071d5cc00
  osc.testfs-OST0007-osc-ffff881071d5cc00
  osc.testfs-OST0008-osc-ffff881071d5cc00
  

  In this example, information about OST connections available on a client is displayed (indicated by "osc").

• To see multiple levels of parameters, use multiple wildcards:

  # lctl list_param osc.*.*
  osc.testfs-OST0000-osc-ffff881071d5cc00.active
  osc.testfs-OST0000-osc-ffff881071d5cc00.blocksize
  osc.testfs-OST0000-osc-ffff881071d5cc00.checksum_type
  osc.testfs-OST0000-osc-ffff881071d5cc00.checksums
  osc.testfs-OST0000-osc-ffff881071d5cc00.connect_flags
  osc.testfs-OST0000-osc-ffff881071d5cc00.contention_seconds
  osc.testfs-OST0000-osc-ffff881071d5cc00.cur_dirty_bytes
  ...
  osc.testfs-OST0000-osc-ffff881071d5cc00.rpc_stats
  

• To view a specific file, use lctl get_param:

  # lctl get_param osc.lustre-OST0000*.rpc_stats
  

For more information about using lctl, see the section called “Setting Parameters with lctl”.

Data can also be viewed using the cat command with the full path to the file. The form of the cat command is similar to that of the lctl get_param command with some differences. Unfortunately, as the Linux kernel has changed over the years, the location of statistics and parameter files has also changed, which means that the Lustre parameter files may be located in either the /proc directory, in the /sys directory, and/or in the/sys/kernel/debug directory, depending on the kernel version and the Lustre version being used. The lctlcommand insulates scripts from these changes and is preferred over direct file access, unless as part of a high-performance monitoring system. In the cat command:

• Replace the dots in the path with slashes.

• Prepend the path with the appropriate directory component:

  /{proc,sys}/{fs,sys}/{lustre,lnet}
  

For example, an lctl get_param command may look like this:

# lctl get_param osc.*.uuid
osc.testfs-OST0000-osc-ffff881071d5cc00.uuid=594db456-0685-bd16-f59b-e72ee90e9819
osc.testfs-OST0001-osc-ffff881071d5cc00.uuid=594db456-0685-bd16-f59b-e72ee90e9819
...

The equivalent cat command may look like this:

# cat /proc/fs/lustre/osc/*/uuid
594db456-0685-bd16-f59b-e72ee90e9819
594db456-0685-bd16-f59b-e72ee90e9819
...

or like this:

# cat /sys/fs/lustre/osc/*/uuid
594db456-0685-bd16-f59b-e72ee90e9819
594db456-0685-bd16-f59b-e72ee90e9819
...

The llstat utility can be used to monitor some Lustre file system I/O activity over a specified time period. For more details, see the section called “ llstat”.

Some data is imported from attached clients and is available in a directory called exports located in the corresponding per-service directory on a Lustre server. For example:

oss:/root# lctl list_param obdfilter.testfs-OST0000.exports.*
# hash ldlm_stats stats uuid

Identifying Lustre File Systems and Servers

Several parameter files on the MGS list existing Lustre file systems and file system servers. The examples below are for a Lustre file system called testfs with one MDT and three OSTs.

• To view all known Lustre file systems, enter:

  mgs# lctl get_param mgs.*.filesystems
  testfs
  

• To view the names of the servers in a file system in which least one server is running, enter:

  lctl get_param mgs.*.live.<filesystem name>
  

  For example:

  mgs# lctl get_param mgs.*.live.testfs
  fsname: testfs
  flags: 0x20     gen: 45
  testfs-MDT0000
  testfs-OST0000
  testfs-OST0001
  testfs-OST0002 
  
  Secure RPC Config Rules: 
  
  imperative_recovery_state:
      state: startup
      nonir_clients: 0
      nidtbl_version: 6
      notify_duration_total: 0.001000
      notify_duation_max:  0.001000
      notify_count: 4
  

• To list all configured devices on the local node, enter:

  # lctl device_list
  0 UP mgs MGS MGS 11
  1 UP mgc MGC192.168.10.34@tcp 1f45bb57-d9be-2ddb-c0b0-5431a49226705
  2 UP mdt MDS MDS_uuid 3
  3 UP lov testfs-mdtlov testfs-mdtlov_UUID 4
  4 UP mds testfs-MDT0000 testfs-MDT0000_UUID 7
  5 UP osc testfs-OST0000-osc testfs-mdtlov_UUID 5
  6 UP osc testfs-OST0001-osc testfs-mdtlov_UUID 5
  7 UP lov testfs-clilov-ce63ca00 08ac6584-6c4a-3536-2c6d-b36cf9cbdaa04
  8 UP mdc testfs-MDT0000-mdc-ce63ca00 08ac6584-6c4a-3536-2c6d-b36cf9cbdaa05
  9 UP osc testfs-OST0000-osc-ce63ca00 08ac6584-6c4a-3536-2c6d-b36cf9cbdaa05
  10 UP osc testfs-OST0001-osc-ce63ca00 08ac6584-6c4a-3536-2c6d-b36cf9cbdaa05
  

  The information provided on each line includes:

  - Device number

  - Device status (UP, INactive, or STopping)

  - Device name

  - Device UUID

  - Reference count (how many users this device has)

• To display the name of any server, view the device label:

  mds# e2label /dev/sda
  testfs-MDT0000 
  

Tuning Multi-Block Allocation (mballoc)

Capabilities supported by mballoc include:

• Pre-allocation for single files to help to reduce fragmentation.
• Pre-allocation for a group of files to enable packing of small files into large, contiguous chunks.
• Stream allocation to help decrease the seek rate.

The following mballoc tunables are available:

Field	Description
mb_max_to_scan	Maximum number of free chunks that mballoc finds before a final decision to avoid a livelock situation.
mb_min_to_scan	Minimum number of free chunks that mballoc searches before picking the best chunk for allocation. This is useful for small requests to reduce fragmentation of big free chunks.
mb_order2_req	For requests equal to 2^N, where N >= mb_order2_req, a fast search is done using a base 2 buddy allocation service.
mb_small_req	mb_small_req - Defines (in MB) the upper bound of "small requests".mb_large_req - Defines (in MB) the lower bound of "large requests".Requests are handled differently based on size:< mb_small_req - Requests are packed together to form large, aggregated requests.> mb_small_req and < mb_large_req - Requests are primarily allocated linearly.> mb_large_req - Requests are allocated since hard disk seek time is less of a concern in this case.In general, small requests are combined to create larger requests, which are then placed close to one another to minimize the number of seeks required to access the data.
mb_large_req
prealloc_table	A table of values used to preallocate space when a new request is received. By default, the table looks like this:prealloc_table 4 8 16 32 64 128 256 512 1024 2048 When a new request is received, space is preallocated at the next higher increment specified in the table. For example, for requests of less than 4 file system blocks, 4 blocks of space are preallocated; for requests between 4 and 8, 8 blocks are preallocated; and so forthAlthough customized values can be entered in the table, the performance of general usage file systems will not typically be improved by modifying the table (in fact, in ext4 systems, the table values are fixed). However, for some specialized workloads, tuning the prealloc_table values may result in smarter preallocation decisions.
mb_group_prealloc	The amount of space (in kilobytes) preallocated for groups of small requests.

Buddy group cache information found in /sys/fs/ldiskfs/*disk_device*/mb_groups may be useful for assessing on-disk fragmentation. For example:

cat /proc/fs/ldiskfs/loop0/mb_groups 
#group: free free frags first pa [ 2^0 2^1 2^2 2^3 2^4 2^5 2^6 2^7 2^8 2^9 
     2^10 2^11 2^12 2^13] 
#0    : 2936 2936 1     42    0  [ 0   0   0   1   1   1   1   2   0   1 
     2    0    0    0   ]

In this example, the columns show:

• #group number
• Available blocks in the group
• Blocks free on a disk
• Number of free fragments
• First free block in the group
• Number of preallocated chunks (not blocks)
• A series of available chunks of different sizes

Monitoring Lustre File System I/O

A number of system utilities are provided to enable collection of data related to I/O activity in a Lustre file system. In general, the data collected describes:

• Data transfer rates and throughput of inputs and outputs external to the Lustre file system, such as network requests or disk I/O operations performed
• Data about the throughput or transfer rates of internal Lustre file system data, such as locks or allocations.

Note

It is highly recommended that you complete baseline testing for your Lustre file system to determine normal I/O activity for your hardware, network, and system workloads. Baseline data will allow you to easily determine when performance becomes degraded in your system. Two particularly useful baseline statistics are:

• brw_stats – Histogram data characterizing I/O requests to the OSTs. For more details, see the section called “Monitoring the OST Block I/O Stream”.
• rpc_stats – Histogram data showing information about RPCs made by clients. For more details, see the section called “Monitoring the Client RPC Stream”.

Monitoring the Client RPC Stream

The rpc_stats file contains histogram data showing information about remote procedure calls (RPCs) that have been made since this file was last cleared. The histogram data can be cleared by writing any value into the rpc_stats file.

Example:

# lctl get_param osc.testfs-OST0000-osc-ffff810058d2f800.rpc_stats
snapshot_time:            1372786692.389858 (secs.usecs)
read RPCs in flight:      0
write RPCs in flight:     1
dio read RPCs in flight:  0
dio write RPCs in flight: 0
pending write pages:      256
pending read pages:       0

                     read                   write
pages per rpc   rpcs   % cum % |       rpcs   % cum %
1:                 0   0   0   |          0   0   0
2:                 0   0   0   |          1   0   0
4:                 0   0   0   |          0   0   0
8:                 0   0   0   |          0   0   0
16:                0   0   0   |          0   0   0
32:                0   0   0   |          2   0   0
64:                0   0   0   |          2   0   0
128:               0   0   0   |          5   0   0
256:             850 100 100   |      18346  99 100

                     read                   write
rpcs in flight  rpcs   % cum % |       rpcs   % cum %
0:               691  81  81   |       1740   9   9
1:                48   5  86   |        938   5  14
2:                29   3  90   |       1059   5  20
3:                17   2  92   |       1052   5  26
4:                13   1  93   |        920   5  31
5:                12   1  95   |        425   2  33
6:                10   1  96   |        389   2  35
7:                30   3 100   |      11373  61  97
8:                 0   0 100   |        460   2 100

                     read                   write
offset          rpcs   % cum % |       rpcs   % cum %
0:               850 100 100   |      18347  99  99
1:                 0   0 100   |          0   0  99
2:                 0   0 100   |          0   0  99
4:                 0   0 100   |          0   0  99
8:                 0   0 100   |          0   0  99
16:                0   0 100   |          1   0  99
32:                0   0 100   |          1   0  99
64:                0   0 100   |          3   0  99
128:               0   0 100   |          4   0 100

The header information includes:

• snapshot_time - UNIX epoch instant the file was read.
• read RPCs in flight - Number of read RPCs issued by the OSC, but not complete at the time of the snapshot. This value should always be less than or equal to max_rpcs_in_flight.
• write RPCs in flight - Number of write RPCs issued by the OSC, but not complete at the time of the snapshot. This value should always be less than or equal to max_rpcs_in_flight.
• dio read RPCs in flight - Direct I/O (as opposed to block I/O) read RPCs issued but not completed at the time of the snapshot.
• dio write RPCs in flight - Direct I/O (as opposed to block I/O) write RPCs issued but not completed at the time of the snapshot.
• pending write pages - Number of pending write pages that have been queued for I/O in the OSC.
• pending read pages - Number of pending read pages that have been queued for I/O in the OSC.

The tabular data is described in the table below. Each row in the table shows the number of reads or writes (ios) occurring for the statistic, the relative percentage (%) of total reads or writes, and the cumulative percentage (cum %) to that point in the table for the statistic.

Field	Description
pages per RPC	Shows cumulative RPC reads and writes organized according to the number of pages in the RPC. A single page RPC increments the 0: row.
RPCs in flight	Shows the number of RPCs that are pending when an RPC is sent. When the first RPC is sent, the 0: row is incremented. If the first RPC is sent while another RPC is pending, the 1: row is incremented and so on.
offset	The page index of the first page read from or written to the object by the RPC.

Analysis:

This table provides a way to visualize the concurrency of the RPC stream. Ideally, you will see a large clump around the max_rpcs_in_flight value, which shows that the network is being kept busy.

For information about optimizing the client I/O RPC stream, see the section called “Tuning the Client I/O RPC Stream”.

Monitoring Client Activity

The stats file maintains statistics accumulate during typical operation of a client across the VFS interface of the Lustre file system. Only non-zero parameters are displayed in the file.

Client statistics are enabled by default.

Note

Statistics for all mounted file systems can be discovered by entering:

lctl get_param llite.*.stats

Example:

client# lctl get_param llite.*.stats
snapshot_time          1308343279.169704 secs.usecs
dirty_pages_hits       14819716 samples [regs]
dirty_pages_misses     81473472 samples [regs]
read_bytes             36502963 samples [bytes] 1 26843582 55488794
write_bytes            22985001 samples [bytes] 0 125912 3379002
brw_read               2279 samples [pages] 1 1 2270
ioctl                  186749 samples [regs]
open                   3304805 samples [regs]
close                  3331323 samples [regs]
seek                   48222475 samples [regs]
fsync                  963 samples [regs]
truncate               9073 samples [regs]
setxattr               19059 samples [regs]
getxattr               61169 samples [regs]

The statistics can be cleared by echoing an empty string into the stats file or by using the command:

lctl set_param llite.*.stats=0

The statistics displayed are described in the table below.

Entry	Description
snapshot_time	UNIX epoch instant the stats file was read.
dirty_page_hits	The number of write operations that have been satisfied by the dirty page cache. See the section called “Tuning the Client I/O RPC Stream” for more information about dirty cache behavior in a Lustre file system.
dirty_page_misses	The number of write operations that were not satisfied by the dirty page cache.
read_bytes	The number of read operations that have occurred. Three additional parameters are displayed:minThe minimum number of bytes read in a single request since the counter was reset.maxThe maximum number of bytes read in a single request since the counter was reset.sumThe accumulated sum of bytes of all read requests since the counter was reset.
write_bytes	The number of write operations that have occurred. Three additional parameters are displayed:minThe minimum number of bytes written in a single request since the counter was reset.maxThe maximum number of bytes written in a single request since the counter was reset.sumThe accumulated sum of bytes of all write requests since the counter was reset.
brw_read	The number of pages that have been read. Three additional parameters are displayed:minThe minimum number of bytes read in a single block read/write (brw) read request since the counter was reset.maxThe maximum number of bytes read in a single brw read requests since the counter was reset.sumThe accumulated sum of bytes of all brw read requests since the counter was reset.
ioctl	The number of combined file and directory ioctl operations.
open	The number of open operations that have succeeded.
close	The number of close operations that have succeeded.
seek	The number of times seek has been called.
fsync	The number of times fsync has been called.
truncate	The total number of calls to both locked and lockless truncate.
setxattr	The number of times extended attributes have been set.
getxattr	The number of times value(s) of extended attributes have been fetched.

Analysis:

Information is provided about the amount and type of I/O activity is taking place on the client.

Monitoring Client Read-Write Extent Statistics

For in-depth troubleshooting, client read-write extent statistics can be accessed to obtain more detail about read/write I/O extents for the file system or for a particular process.

Note

By default, statistics are not collected in the offset_stats, extents_stats, and extents_stats_per_process files to reduce monitoring overhead when this information is not needed. The collection of statistics in all three of these files is activated by writing anything, except for 0 (zero) and "disable", into any one of the files.

Client-Based I/O Extent Size Survey

The extents_stats histogram in the llite directory shows the statistics for the sizes of the read/write I/O extents. This file does not maintain the per process statistics.

Example:

# lctl get_param llite.testfs-*.extents_stats
snapshot_time:                     1213828728.348516 (secs.usecs)
                       read           |            write
extents          calls  %      cum%   |     calls  %     cum%

0K - 4K :        0      0      0      |     2      2     2
4K - 8K :        0      0      0      |     0      0     2
8K - 16K :       0      0      0      |     0      0     2
16K - 32K :      0      0      0      |     20     23    26
32K - 64K :      0      0      0      |     0      0     26
64K - 128K :     0      0      0      |     51     60    86
128K - 256K :    0      0      0      |     0      0     86
256K - 512K :    0      0      0      |     0      0     86
512K - 1024K :   0      0      0      |     0      0     86
1M - 2M :        0      0      0      |     11     13    100

In this example, snapshot_time is the UNIX epoch instant the file was read. The table shows cumulative extents organized according to size with statistics provided separately for reads and writes. Each row in the table shows the number of RPCs for reads and writes respectively (calls), the relative percentage of total calls (%), and the cumulative percentage to that point in the table of calls (cum %).

The file can be cleared by issuing the following command:

# lctl set_param llite.testfs-*.extents_stats=1

Per-Process Client I/O Statistics

The extents_stats_per_process file maintains the I/O extent size statistics on a per-process basis.

Example:

# lctl get_param llite.testfs-*.extents_stats_per_process
snapshot_time:                     1213828762.204440 (secs.usecs)
                          read            |             write
extents            calls   %      cum%    |      calls   %       cum%
 
PID: 11488
   0K - 4K :       0       0       0      |      0       0       0
   4K - 8K :       0       0       0      |      0       0       0
   8K - 16K :      0       0       0      |      0       0       0
   16K - 32K :     0       0       0      |      0       0       0
   32K - 64K :     0       0       0      |      0       0       0
   64K - 128K :    0       0       0      |      0       0       0
   128K - 256K :   0       0       0      |      0       0       0
   256K - 512K :   0       0       0      |      0       0       0
   512K - 1024K :  0       0       0      |      0       0       0
   1M - 2M :       0       0       0      |      10      100     100
 
PID: 11491
   0K - 4K :       0       0       0      |      0       0       0
   4K - 8K :       0       0       0      |      0       0       0
   8K - 16K :      0       0       0      |      0       0       0
   16K - 32K :     0       0       0      |      20      100     100
   
PID: 11424
   0K - 4K :       0       0       0      |      0       0       0
   4K - 8K :       0       0       0      |      0       0       0
   8K - 16K :      0       0       0      |      0       0       0
   16K - 32K :     0       0       0      |      0       0       0
   32K - 64K :     0       0       0      |      0       0       0
   64K - 128K :    0       0       0      |      16      100     100
 
PID: 11426
   0K - 4K :       0       0       0      |      1       100     100
 
PID: 11429
   0K - 4K :       0       0       0      |      1       100     100
 

This table shows cumulative extents organized according to size for each process ID (PID) with statistics provided separately for reads and writes. Each row in the table shows the number of RPCs for reads and writes respectively (calls), the relative percentage of total calls (%), and the cumulative percentage to that point in the table of calls (cum %).

Monitoring the OST Block I/O Stream

The brw_stats file in the obdfilter directory contains histogram data showing statistics for number of I/O requests sent to the disk, their size, and whether they are contiguous on the disk or not.

Example:

Enter on the OSS:

# lctl get_param obdfilter.testfs-OST0000.brw_stats 
snapshot_time:         1372775039.769045 (secs.usecs)
                           read      |      write
pages per bulk r/w     rpcs  % cum % |  rpcs   % cum %
1:                     108 100 100   |    39   0   0
2:                       0   0 100   |     6   0   0
4:                       0   0 100   |     1   0   0
8:                       0   0 100   |     0   0   0
16:                      0   0 100   |     4   0   0
32:                      0   0 100   |    17   0   0
64:                      0   0 100   |    12   0   0
128:                     0   0 100   |    24   0   0
256:                     0   0 100   | 23142  99 100

                           read      |      write
discontiguous pages    rpcs  % cum % |  rpcs   % cum %
0:                     108 100 100   | 23245 100 100

                           read      |      write
discontiguous blocks   rpcs  % cum % |  rpcs   % cum %
0:                     108 100 100   | 23243  99  99
1:                       0   0 100   |     2   0 100

                           read      |      write
disk fragmented I/Os   ios   % cum % |   ios   % cum %
0:                      94  87  87   |     0   0   0
1:                      14  12 100   | 23243  99  99
2:                       0   0 100   |     2   0 100

                           read      |      write
disk I/Os in flight    ios   % cum % |   ios   % cum %
1:                      14 100 100   | 20896  89  89
2:                       0   0 100   |  1071   4  94
3:                       0   0 100   |   573   2  96
4:                       0   0 100   |   300   1  98
5:                       0   0 100   |   166   0  98
6:                       0   0 100   |   108   0  99
7:                       0   0 100   |    81   0  99
8:                       0   0 100   |    47   0  99
9:                       0   0 100   |     5   0 100

                           read      |      write
I/O time (1/1000s)     ios   % cum % |   ios   % cum %
1:                      94  87  87   |     0   0   0
2:                       0   0  87   |     7   0   0
4:                      14  12 100   |    27   0   0
8:                       0   0 100   |    14   0   0
16:                      0   0 100   |    31   0   0
32:                      0   0 100   |    38   0   0
64:                      0   0 100   | 18979  81  82
128:                     0   0 100   |   943   4  86
256:                     0   0 100   |  1233   5  91
512:                     0   0 100   |  1825   7  99
1K:                      0   0 100   |   99   0  99
2K:                      0   0 100   |     0   0  99
4K:                      0   0 100   |     0   0  99
8K:                      0   0 100   |    49   0 100

                           read      |      write
disk I/O size          ios   % cum % |   ios   % cum %
4K:                     14 100 100   |    41   0   0
8K:                      0   0 100   |     6   0   0
16K:                     0   0 100   |     1   0   0
32K:                     0   0 100   |     0   0   0
64K:                     0   0 100   |     4   0   0
128K:                    0   0 100   |    17   0   0
256K:                    0   0 100   |    12   0   0
512K:                    0   0 100   |    24   0   0
1M:                      0   0 100   | 23142  99 100

The tabular data is described in the table below. Each row in the table shows the number of reads and writes occurring for the statistic (ios), the relative percentage of total reads or writes (%), and the cumulative percentage to that point in the table for the statistic (cum %).

Field	Description
pages per bulk r/w	Number of pages per RPC request, which should match aggregate client rpc_stats (see the section called “Monitoring the Client RPC Stream”).
discontiguous pages	Number of discontinuities in the logical file offset of each page in a single RPC.
discontiguous blocks	Number of discontinuities in the physical block allocation in the file system for a single RPC.
disk fragmented I/Os	Number of I/Os that were not written entirely sequentially.
disk I/Os in flight	Number of disk I/Os currently pending.
I/O time (1/1000s)	Amount of time for each I/O operation to complete.
disk I/O size	Size of each I/O operation.

Analysis:

This data provides an indication of extent size and distribution in the file system.

Tuning Lustre File System I/O

Each OSC has its own tree of tunables. For example:

$ lctl lctl list_param osc.*.*
osc.myth-OST0000-osc-ffff8804296c2800.active
osc.myth-OST0000-osc-ffff8804296c2800.blocksize
osc.myth-OST0000-osc-ffff8804296c2800.checksum_dump
osc.myth-OST0000-osc-ffff8804296c2800.checksum_type
osc.myth-OST0000-osc-ffff8804296c2800.checksums
osc.myth-OST0000-osc-ffff8804296c2800.connect_flags
:
:
osc.myth-OST0000-osc-ffff8804296c2800.state
osc.myth-OST0000-osc-ffff8804296c2800.stats
osc.myth-OST0000-osc-ffff8804296c2800.timeouts
osc.myth-OST0000-osc-ffff8804296c2800.unstable_stats
osc.myth-OST0000-osc-ffff8804296c2800.uuid
osc.myth-OST0001-osc-ffff8804296c2800.active
osc.myth-OST0001-osc-ffff8804296c2800.blocksize
osc.myth-OST0001-osc-ffff8804296c2800.checksum_dump
osc.myth-OST0001-osc-ffff8804296c2800.checksum_type
:
:

The following sections describe some of the parameters that can be tuned in a Lustre file system.

Tuning the Client I/O RPC Stream

Ideally, an optimal amount of data is packed into each I/O RPC and a consistent number of issued RPCs are in progress at any time. To help optimize the client I/O RPC stream, several tuning variables are provided to adjust behavior according to network conditions and cluster size. For information about monitoring the client I/O RPC stream, see the section called “Monitoring the Client RPC Stream”.

RPC stream tunables include:

• osc.*osc_instance*.checksums - Controls whether the client will calculate data integrity checksums for the bulk data transferred to the OST. Data integrity checksums are enabled by default. The algorithm used can be set using the checksum_type parameter.

• osc.*osc_instance*.checksum_type - Controls the data integrity checksum algorithm used by the client. The available algorithms are determined by the set of algorihtms. The checksum algorithm used by default is determined by first selecting the fastest algorithms available on the OST, and then selecting the fastest of those algorithms on the client, which depends on available optimizations in the CPU hardware and kernel. The default algorithm can be overridden by writing the algorithm name into the checksum_type parameter. Available checksum types can be seen on the client by reading the checksum_type parameter. Currently supported checksum types are: adler, crc32, crc32c

  Introduced in Lustre 2.12In Lustre release 2.12 additional checksum types were added to allow end-to-end checksum integration with T10-PI capable hardware. The client will compute the appropriate checksum type, based on the checksum type used by the storage, for the RPC checksum, which will be verified by the server and passed on to the storage. The T10-PI checksum types are: t10ip512, t10ip4K, t10crc512, t10crc4K

• osc.*osc_instance*.max_dirty_mb - Controls how many MiB of dirty data can be written into the client pagecache for writes by each OSC. When this limit is reached, additional writes block until previously-cached data is written to the server. This may be changed by the lctl set_param command. Only values larger than 0 and smaller than the lesser of 2048 MiB or 1/4 of client RAM are valid. Performance can suffers if the client cannot aggregate enough data per OSC to form a full RPC (as set by the max_pages_per_rpc) parameter, unless the application is doing very large writes itself.

  To maximize performance, the value for max_dirty_mb is recommended to be at least 4 * max_pages_per_rpc * max_rpcs_in_flight.

• osc.*osc_instance*.cur_dirty_bytes - A read-only value that returns the current number of bytes written and cached by this OSC.

• osc.*osc_instance*.max_pages_per_rpc - The maximum number of pages that will be sent in a single RPC request to the OST. The minimum value is one page and the maximum value is 16 MiB (4096 on systems with PAGE_SIZE of 4 KiB), with the default value of 4 MiB in one RPC. The upper limit may also be constrained by ofd.*.brw_size setting on the OSS, and applies to all clients connected to that OST. It is also possible to specify a units suffix (e.g. max_pages_per_rpc=4M), so the RPC size can be set independently of the client PAGE_SIZE.

• osc.*osc_instance*.max_rpcs_in_flight - The maximum number of concurrent RPCs in flight from an OSC to its OST. If the OSC tries to initiate an RPC but finds that it already has the same number of RPCs outstanding, it will wait to issue further RPCs until some complete. The minimum setting is 1 and maximum setting is 256. The default value is 8 RPCs.

  To improve small file I/O performance, increase the max_rpcs_in_flight value.

• llite.*fsname_instance*.max_cache_mb - Maximum amount of inactive data cached by the client. The default value is 3/4 of the client RAM.

Note

The value for *osc_instance* and *fsname_instance* are unique to each mount point to allow associating osc, mdc, lov, lmv, and llite parameters with the same mount point. However, it is common for scripts to use a wildcard * or a filesystem-specific wildcard *fsname-\** to specify the parameter settings uniformly on all clients. For example:

client$ lctl get_param osc.testfs-OST0000*.rpc_stats
osc.testfs-OST0000-osc-ffff88107412f400.rpc_stats=
snapshot_time:         1375743284.337839 (secs.usecs)
read RPCs in flight:  0
write RPCs in flight: 0

Tuning File Readahead and Directory Statahead

File readahead and directory statahead enable reading of data into memory before a process requests the data. File readahead prefetches file content data into memory for read() related calls, while directory statahead fetches file metadata into memory for readdir() and stat() related calls. When readahead and statahead work well, a process that accesses data finds that the information it needs is available immediately in memory on the client when requested without the delay of network I/O.

Tuning File Readahead

File readahead is triggered when two or more sequential reads by an application fail to be satisfied by data in the Linux buffer cache. The size of the initial readahead is determined by the RPC size and the file stripe size, but will typically be at least 1 MiB. Additional readaheads grow linearly and increment until the per-file or per-system readahead cache limit on the client is reached.

Readahead tunables include:

• llite.*fsname_instance*.max_read_ahead_mb - Controls the maximum amount of data readahead on a file. Files are read ahead in RPC-sized chunks (4 MiB, or the size of the read() call, if larger) after the second sequential read on a file descriptor. Random reads are done at the size of the read() call only (no readahead). Reads to non-contiguous regions of the file reset the readahead algorithm, and readahead is not triggered until sequential reads take place again.

  This is the global limit for all files and cannot be larger than 1/2 of the client RAM. To disable readahead, setmax_read_ahead_mb=0.

• llite.*fsname_instance*.max_read_ahead_per_file_mb - Controls the maximum number of megabytes (MiB) of data that should be prefetched by the client when sequential reads are detected on a file. This is the per-file readahead limit and cannot be larger than max_read_ahead_mb.

• llite.*fsname_instance*.max_read_ahead_whole_mb - Controls the maximum size of a file in MiB that is read in its entirety upon access, regardless of the size of the read() call. This avoids multiple small read RPCs on relatively small files, when it is not possible to efficiently detect a sequential read pattern before the whole file has been read.

  The default value is the greater of 2 MiB or the size of one RPC, as given by max_pages_per_rpc.

Tuning Directory Statahead and AGL

Many system commands, such as ls –l, du, and find, traverse a directory sequentially. To make these commands run efficiently, the directory statahead can be enabled to improve the performance of directory traversal.

The statahead tunables are:

• statahead_max - Controls the maximum number of file attributes that will be prefetched by the statahead thread. By default, statahead is enabled and statahead_max is 32 files.

  To disable statahead, set statahead_max to zero via the following command on the client:

  lctl set_param llite.*.statahead_max=0
  

  To change the maximum statahead window size on a client:

  lctl set_param llite.*.statahead_max=n
  

  The maximum statahead_max is 8192 files.

  The directory statahead thread will also prefetch the file size/block attributes from the OSTs, so that all file attributes are available on the client when requested by an application. This is controlled by the asynchronous glimpse lock (AGL) setting. The AGL behaviour can be disabled by setting:

  lctl set_param llite.*.statahead_agl=0
  

• statahead_stats - A read-only interface that provides current statahead and AGL statistics, such as how many times statahead/AGL has been triggered since the last mount, how many statahead/AGL failures have occurred due to an incorrect prediction or other causes.

  Note

  AGL behaviour is affected by statahead since the inodes processed by AGL are built by the statahead thread. If statahead is disabled, then AGL is also disabled.

Tuning OSS Read Cache

The OSS read cache feature provides read-only caching of data on an OSS. This functionality uses the Linux page cache to store the data and uses as much physical memory as is allocated.

OSS read cache improves Lustre file system performance in these situations:

• Many clients are accessing the same data set (as in HPC applications or when diskless clients boot from the Lustre file system).
• One client is storing data while another client is reading it (i.e., clients are exchanging data via the OST).
• A client has very limited caching of its own.

OSS read cache offers these benefits:

• Allows OSTs to cache read data more frequently.
• Improves repeated reads to match network speeds instead of disk speeds.
• Provides the building blocks for OST write cache (small-write aggregation).

Using OSS Read Cache

OSS read cache is implemented on the OSS, and does not require any special support on the client side. Since OSS read cache uses the memory available in the Linux page cache, the appropriate amount of memory for the cache should be determined based on I/O patterns; if the data is mostly reads, then more cache is required than would be needed for mostly writes.

OSS read cache is managed using the following tunables:

• read_cache_enable - Controls whether data read from disk during a read request is kept in memory and available for later read requests for the same data, without having to re-read it from disk. By default, read cache is enabled (read_cache_enable=1).

  When the OSS receives a read request from a client, it reads data from disk into its memory and sends the data as a reply to the request. If read cache is enabled, this data stays in memory after the request from the client has been fulfilled. When subsequent read requests for the same data are received, the OSS skips reading data from disk and the request is fulfilled from the cached data. The read cache is managed by the Linux kernel globally across all OSTs on that OSS so that the least recently used cache pages are dropped from memory when the amount of free memory is running low.

  If read cache is disabled (read_cache_enable=0), the OSS discards the data after a read request from the client is serviced and, for subsequent read requests, the OSS again reads the data from disk.

  To disable read cache on all the OSTs of an OSS, run:

  root@oss1# lctl set_param obdfilter.*.read_cache_enable=0
  

  To re-enable read cache on one OST, run:

  root@oss1# lctl set_param obdfilter.{OST_name}.read_cache_enable=1
  

  To check if read cache is enabled on all OSTs on an OSS, run:

  root@oss1# lctl get_param obdfilter.*.read_cache_enable
  

• writethrough_cache_enable - Controls whether data sent to the OSS as a write request is kept in the read cache and available for later reads, or if it is discarded from cache when the write is completed. By default, the writethrough cache is enabled (writethrough_cache_enable=1).

  When the OSS receives write requests from a client, it receives data from the client into its memory and writes the data to disk. If the writethrough cache is enabled, this data stays in memory after the write request is completed, allowing the OSS to skip reading this data from disk if a later read request, or partial-page write request, for the same data is received.

  If the writethrough cache is disabled (writethrough_cache_enabled=0), the OSS discards the data after the write request from the client is completed. For subsequent read requests, or partial-page write requests, the OSS must re-read the data from disk.

  Enabling writethrough cache is advisable if clients are doing small or unaligned writes that would cause partial-page updates, or if the files written by one node are immediately being accessed by other nodes. Some examples where enabling writethrough cache might be useful include producer-consumer I/O models or shared-file writes with a different node doing I/O not aligned on 4096-byte boundaries.

  Disabling the writethrough cache is advisable when files are mostly written to the file system but are not re-read within a short time period, or files are only written and re-read by the same node, regardless of whether the I/O is aligned or not.

  To disable the writethrough cache on all OSTs of an OSS, run:

  root@oss1# lctl set_param obdfilter.*.writethrough_cache_enable=0
  

  To re-enable the writethrough cache on one OST, run:

  root@oss1# lctl set_param obdfilter.{OST_name}.writethrough_cache_enable=1
  

  To check if the writethrough cache is enabled, run:

  root@oss1# lctl get_param obdfilter.*.writethrough_cache_enable
  

• readcache_max_filesize - Controls the maximum size of a file that both the read cache and writethrough cache will try to keep in memory. Files larger than readcache_max_filesize will not be kept in cache for either reads or writes.

  Setting this tunable can be useful for workloads where relatively small files are repeatedly accessed by many clients, such as job startup files, executables, log files, etc., but large files are read or written only once. By not putting the larger files into the cache, it is much more likely that more of the smaller files will remain in cache for a longer time.

  When setting readcache_max_filesize, the input value can be specified in bytes, or can have a suffix to indicate other binary units such as K (kilobytes), M (megabytes), G (gigabytes), T (terabytes), or P(petabytes).

  To limit the maximum cached file size to 32 MB on all OSTs of an OSS, run:

  root@oss1# lctl set_param obdfilter.*.readcache_max_filesize=32M
  

  To disable the maximum cached file size on an OST, run:

  root@oss1# lctl set_param obdfilter.{OST_name}.readcache_max_filesize=-1
  

  To check the current maximum cached file size on all OSTs of an OSS, run:

  root@oss1# lctl get_param obdfilter.*.readcache_max_filesize
  

Enabling OSS Asynchronous Journal Commit

The OSS asynchronous journal commit feature asynchronously writes data to disk without forcing a journal flush. This reduces the number of seeks and significantly improves performance on some hardware.

Note

Asynchronous journal commit cannot work with direct I/O-originated writes (O_DIRECT flag set). In this case, a journal flush is forced.

When the asynchronous journal commit feature is enabled, client nodes keep data in the page cache (a page reference). Lustre clients monitor the last committed transaction number (transno) in messages sent from the OSS to the clients. When a client sees that the last committed transno reported by the OSS is at least equal to the bulk write transno, it releases the reference on the corresponding pages. To avoid page references being held for too long on clients after a bulk write, a 7 second ping request is scheduled (the default OSS file system commit time interval is 5 seconds) after the bulk write reply is received, so the OSS has an opportunity to report the last committed transno.

If the OSS crashes before the journal commit occurs, then intermediate data is lost. However, OSS recovery functionality incorporated into the asynchronous journal commit feature causes clients to replay their write requests and compensate for the missing disk updates by restoring the state of the file system.

By default, sync_journal is enabled (sync_journal=1), so that journal entries are committed synchronously. To enable asynchronous journal commit, set the sync_journal parameter to 0 by entering:

$ lctl set_param obdfilter.*.sync_journal=0 
obdfilter.lol-OST0001.sync_journal=0

An associated sync-on-lock-cancel feature (enabled by default) addresses a data consistency issue that can result if an OSS crashes after multiple clients have written data into intersecting regions of an object, and then one of the clients also crashes. A condition is created in which the POSIX requirement for continuous writes is violated along with a potential for corrupted data. With sync-on-lock-cancel enabled, if a cancelled lock has any volatile writes attached to it, the OSS synchronously writes the journal to disk on lock cancellation. Disabling the sync-on-lock-cancel feature may enhance performance for concurrent write workloads, but it is recommended that you not disable this feature.

The sync_on_lock_cancel parameter can be set to the following values:

• always - Always force a journal flush on lock cancellation (default when async_journal is enabled).
• blocking - Force a journal flush only when the local cancellation is due to a blocking callback.
• never - Do not force any journal flush (default when async_journal is disabled).

For example, to set sync_on_lock_cancel to not to force a journal flush, use a command similar to:

$ lctl get_param obdfilter.*.sync_on_lock_cancel
obdfilter.lol-OST0001.sync_on_lock_cancel=never

Introduced in Lustre 2.8

Tuning the Client Metadata RPC Stream

The client metadata RPC stream represents the metadata RPCs issued in parallel by a client to a MDT target. The metadata RPCs can be split in two categories: the requests that do not modify the file system (like getattr operation), and the requests that do modify the file system (like create, unlink, setattr operations). To help optimize the client metadata RPC stream, several tuning variables are provided to adjust behavior according to network conditions and cluster size.

Note that increasing the number of metadata RPCs issued in parallel might improve the performance metadata intensive parallel applications, but as a consequence it will consume more memory on the client and on the MDS.

Configuring the Client Metadata RPC Stream

The MDC max_rpcs_in_flight parameter defines the maximum number of metadata RPCs, both modifying and non-modifying RPCs, that can be sent in parallel by a client to a MDT target. This includes every file system metadata operations, such as file or directory stat, creation, unlink. The default setting is 8, minimum setting is 1 and maximum setting is 256.

To set the max_rpcs_in_flight parameter, run the following command on the Lustre client:

client$ lctl set_param mdc.*.max_rpcs_in_flight=16

The MDC max_mod_rpcs_in_flight parameter defines the maximum number of file system modifying RPCs that can be sent in parallel by a client to a MDT target. For example, the Lustre client sends modify RPCs when it performs file or directory creation, unlink, access permission modification or ownership modification. The default setting is 7, minimum setting is 1 and maximum setting is 256.

To set the max_mod_rpcs_in_flight parameter, run the following command on the Lustre client:

client$ lctl set_param mdc.*.max_mod_rpcs_in_flight=12

The max_mod_rpcs_in_flight value must be strictly less than the max_rpcs_in_flight value. It must also be less or equal to the MDT max_mod_rpcs_per_client value. If one of theses conditions is not enforced, the setting fails and an explicit message is written in the Lustre log.

The MDT max_mod_rpcs_per_client parameter is a tunable of the kernel module mdt that defines the maximum number of file system modifying RPCs in flight allowed per client. The parameter can be updated at runtime, but the change is effective to new client connections only. The default setting is 8.

To set the max_mod_rpcs_per_client parameter, run the following command on the MDS:

mds$ echo 12 > /sys/module/mdt/parameters/max_mod_rpcs_per_client

Monitoring the Client Metadata RPC Stream

The rpc_stats file contains histogram data showing information about modify metadata RPCs. It can be helpful to identify the level of parallelism achieved by an application doing modify metadata operations.

Example:

client$ lctl get_param mdc.*.rpc_stats
snapshot_time:         1441876896.567070 (secs.usecs)
modify_RPCs_in_flight:  0

                        modify
rpcs in flight        rpcs   % cum %
0:                       0   0   0
1:                      56   0   0
2:                      40   0   0
3:                      70   0   0
4                       41   0   0
5:                      51   0   1
6:                      88   0   1
7:                     366   1   2
8:                    1321   5   8
9:                    3624  15  23
10:                   6482  27  50
11:                   7321  30  81
12:                   4540  18 100

The file information includes:

• snapshot_time - UNIX epoch instant the file was read.
• modify_RPCs_in_flight - Number of modify RPCs issued by the MDC, but not completed at the time of the snapshot. This value should always be less than or equal to max_mod_rpcs_in_flight.
• rpcs in flight - Number of modify RPCs that are pending when a RPC is sent, the relative percentage (%) of total modify RPCs, and the cumulative percentage (cum %) to that point.

If a large proportion of modify metadata RPCs are issued with a number of pending metadata RPCs close to themax_mod_rpcs_in_flight value, it means the max_mod_rpcs_in_flight value could be increased to improve the modify metadata performance.

Configuring Timeouts in a Lustre File System

In a Lustre file system, RPC timeouts are set using an adaptive timeouts mechanism, which is enabled by default. Servers track RPC completion times and then report back to clients estimates for completion times for future RPCs. Clients use these estimates to set RPC timeout values. If the processing of server requests slows down for any reason, the server estimates for RPC completion increase, and clients then revise RPC timeout values to allow more time for RPC completion.

If the RPCs queued on the server approach the RPC timeout specified by the client, to avoid RPC timeouts and disconnect/reconnect cycles, the server sends an "early reply" to the client, telling the client to allow more time. Conversely, as server processing speeds up, RPC timeout values decrease, resulting in faster detection if the server becomes non-responsive and quicker connection to the failover partner of the server.

Configuring Adaptive Timeouts

The adaptive timeout parameters in the table below can be set persistently system-wide using lctl conf_param on the MGS. For example, the following command sets the at_max value for all servers and clients associated with the file system testfs:

lctl conf_param testfs.sys.at_max=1500

Note

Clients that access multiple Lustre file systems must use the same parameter values for all file systems.

Parameter	Description
at_min	Minimum adaptive timeout (in seconds). The default value is 0. The at_min parameter is the minimum processing time that a server will report. Ideally, at_min should be set to its default value. Clients base their timeouts on this value, but they do not use this value directly.If, for unknown reasons (usually due to temporary network outages), the adaptive timeout value is too short and clients time out their RPCs, you can increase the at_min value to compensate for this.
at_max	Maximum adaptive timeout (in seconds). The at_max parameter is an upper-limit on the service time estimate. If at_max is reached, an RPC request times out.Setting at_max to 0 causes adaptive timeouts to be disabled and a fixed timeout method to be used instead (see the section called “Setting Static Timeouts” Note If slow hardware causes the service estimate to increase beyond the default value of at_max, increase at_max to the maximum time you are willing to wait for an RPC completion.
at_history	Time period (in seconds) within which adaptive timeouts remember the slowest event that occurred. The default is 600.
at_early_margin	Amount of time before the Lustre server sends an early reply (in seconds). Default is 5.
at_extra	Incremental amount of time that a server requests with each early reply (in seconds). The server does not know how much time the RPC will take, so it asks for a fixed value. The default is 30, which provides a balance between sending too many early replies for the same RPC and overestimating the actual completion time.When a server finds a queued request about to time out and needs to send an early reply out, the server adds the at_extra value. If the time expires, the Lustre server drops the request, and the client enters recovery status and reconnects to restore the connection to normal status.If you see multiple early replies for the same RPC asking for 30-second increases, change the at_extra value to a larger number to cut down on early replies sent and, therefore, network load.
ldlm_enqueue_min	Minimum lock enqueue time (in seconds). The default is 100. The time it takes to enqueue a lock, ldlm_enqueue, is the maximum of the measured enqueue estimate (influenced by at_min and at_max parameters), multiplied by a weighting factor and the value of ldlm_enqueue_min.Lustre Distributed Lock Manager (LDLM) lock enqueues have a dedicated minimum value for ldlm_enqueue_min. Lock enqueue timeouts increase as the measured enqueue times increase (similar to adaptive timeouts).

Interpreting Adaptive Timeout Information

Adaptive timeout information can be obtained via lctl get_param {osc,mdc}.*.timeouts files on each client and lctl get_param {ost,mds}.*.*.timeouts on each server. To read information from a timeouts file, enter a command similar to:

# lctl get_param -n ost.*.ost_io.timeouts
service : cur 33  worst 34 (at 1193427052, 1600s ago) 1 1 33 2

In this example, the ost_io service on this node is currently reporting an estimated RPC service time of 33 seconds. The worst RPC service time was 34 seconds, which occurred 26 minutes ago.

The output also provides a history of service times. Four "bins" of adaptive timeout history are shown, with the maximum RPC time in each bin reported. In both the 0-150s bin and the 150-300s bin, the maximum RPC time was 1. The 300-450s bin shows the worst (maximum) RPC time at 33 seconds, and the 450-600s bin shows a maximum of RPC time of 2 seconds. The estimated service time is the maximum value in the four bins (33 seconds in this example).

Service times (as reported by the servers) are also tracked in the client OBDs, as shown in this example:

# lctl get_param osc.*.timeouts
last reply : 1193428639, 0d0h00m00s ago
network    : cur  1 worst  2 (at 1193427053, 0d0h26m26s ago)  1  1  1  1
portal 6   : cur 33 worst 34 (at 1193427052, 0d0h26m27s ago) 33 33 33  2
portal 28  : cur  1 worst  1 (at 1193426141, 0d0h41m38s ago)  1  1  1  1
portal 7   : cur  1 worst  1 (at 1193426141, 0d0h41m38s ago)  1  0  1  1
portal 17  : cur  1 worst  1 (at 1193426177, 0d0h41m02s ago)  1  0  0  1

In this example, portal 6, the ost_io service portal, shows the history of service estimates reported by the portal.

Server statistic files also show the range of estimates including min, max, sum, and sum-squared. For example:

# lctl get_param mdt.*.mdt.stats
...
req_timeout               6 samples [sec] 1 10 15 105
...

Setting Static Timeouts

The Lustre software provides two sets of static (fixed) timeouts, LND timeouts and Lustre timeouts, which are used when adaptive timeouts are not enabled.

• LND timeouts - LND timeouts ensure that point-to-point communications across a network complete in a finite time in the presence of failures, such as packages lost or broken connections. LND timeout parameters are set for each individual LND.

  LND timeouts are logged with the S_LND flag set. They are not printed as console messages, so check the Lustre log for D_NETERROR messages or enable printing of D_NETERROR messages to the console using:

  lctl set_param printk=+neterror
  

  Congested routers can be a source of spurious LND timeouts. To avoid this situation, increase the number of LNet router buffers to reduce back-pressure and/or increase LND timeouts on all nodes on all connected networks. Also consider increasing the total number of LNet router nodes in the system so that the aggregate router bandwidth matches the aggregate server bandwidth.

• Lustre timeouts - Lustre timeouts ensure that Lustre RPCs complete in a finite time in the presence of failures when adaptive timeouts are not enabled. Adaptive timeouts are enabled by default. To disable adaptive timeouts at run time, set at_max to 0 by running on the MGS:

  # lctl conf_param fsname.sys.at_max=0
  

  Note

  Changing the status of adaptive timeouts at runtime may cause a transient client timeout, recovery, and reconnection.

  Lustre timeouts are always printed as console messages.

  If Lustre timeouts are not accompanied by LND timeouts, increase the Lustre timeout on both servers and clients. Lustre timeouts are set using a command such as the following:

  # lctl set_param timeout=30
  

  Lustre timeout parameters are described in the table below.

Parameter	Description
timeout	The time that a client waits for a server to complete an RPC (default 100s). Servers wait half this time for a normal client RPC to complete and a quarter of this time for a single bulk request (read or write of up to 4 MB) to complete. The client pings recoverable targets (MDS and OSTs) at one quarter of the timeout, and the server waits one and a half times the timeout before evicting a client for being "stale."Lustre client sends periodic 'ping' messages to servers with which it has had no communication for the specified period of time. Any network activity between a client and a server in the file system also serves as a ping.
ldlm_timeout	The time that a server waits for a client to reply to an initial AST (lock cancellation request). The default is 20s for an OST and 6s for an MDS. If the client replies to the AST, the server will give it a normal timeout (half the client timeout) to flush any dirty data and release the lock.
fail_loc	An internal debugging failure hook. The default value of 0 means that no failure will be triggered or injected.
dump_on_timeout	Triggers a dump of the Lustre debug log when a timeout occurs. The default value of 0 (zero) means a dump of the Lustre debug log will not be triggered.
dump_on_eviction	Triggers a dump of the Lustre debug log when an eviction occurs. The default value of 0(zero) means a dump of the Lustre debug log will not be triggered.

Monitoring LNet

LNet information is located via lctl get_param in these parameters:

• peers - Shows all NIDs known to this node and provides information on the queue state.

  Example:

  # lctl get_param peers
  nid                refs   state  max  rtr  min   tx    min   queue
  0@lo               1      ~rtr   0    0    0     0     0     0
  192.168.10.35@tcp  1      ~rtr   8    8    8     8     6     0
  192.168.10.36@tcp  1      ~rtr   8    8    8     8     6     0
  192.168.10.37@tcp  1      ~rtr   8    8    8     8     6     0
  

  The fields are explained in the table below:

  Field	Description
  refs	A reference count.
  state	If the node is a router, indicates the state of the router. Possible values are:NA - Indicates the node is not a router.up/down- Indicates if the node (router) is up or down.
  max	Maximum number of concurrent sends from this peer.
  rtr	Number of routing buffer credits.
  min	Minimum number of routing buffer credits seen.
  tx	Number of send credits.
  min	Minimum number of send credits seen.
  queue	Total bytes in active/queued sends.

  Credits are initialized to allow a certain number of operations (in the example above the table, eight as shown in the max column. LNet keeps track of the minimum number of credits ever seen over time showing the peak congestion that has occurred during the time monitored. Fewer available credits indicates a more congested resource.

  The number of credits currently in flight (number of transmit credits) is shown in the tx column. The maximum number of send credits available is shown in the max column and never changes. The number of router buffers available for consumption by a peer is shown in the rtr column.

  Therefore, rtr – tx is the number of transmits in flight. Typically, rtr == max, although a configuration can be set such that max >= rtr. The ratio of routing buffer credits to send credits (rtr/tx) that is less than maxindicates operations are in progress. If the ratio rtr/tx is greater than max, operations are blocking.

  LNet also limits concurrent sends and number of router buffers allocated to a single peer so that no peer can occupy all these resources.

• nis - Shows the current queue health on this node.

  Example:

  # lctl get_param nis
  nid                    refs   peer    max   tx    min
  0@lo                   3      0       0     0     0
  192.168.10.34@tcp      4      8       256   256   252
  

  The fields are explained in the table below.

  Field	Description
  nid	Network interface.
  refs	Internal reference counter.
  peer	Number of peer-to-peer send credits on this NID. Credits are used to size buffer pools.
  max	Total number of send credits on this NID.
  tx	Current number of send credits available on this NID.
  min	Lowest number of send credits available on this NID.
  queue	Total bytes in active/queued sends.

  Analysis:

  Subtracting max from tx (max - tx) yields the number of sends currently active. A large or increasing number of active sends may indicate a problem.

Allocating Free Space on OSTs

Free space is allocated using either a round-robin or a weighted algorithm. The allocation method is determined by the maximum amount of free-space imbalance between the OSTs. When free space is relatively balanced across OSTs, the faster round-robin allocator is used, which maximizes network balancing. The weighted allocator is used when any two OSTs are out of balance by more than a specified threshold.

Free space distribution can be tuned using these two tunable parameters:

• lod.*.qos_threshold_rr - The threshold at which the allocation method switches from round-robin to weighted is set in this file. The default is to switch to the weighted algorithm when any two OSTs are out of balance by more than 17 percent.
• lod.*.qos_prio_free - The weighting priority used by the weighted allocator can be adjusted in this file. Increasing the value of qos_prio_free puts more weighting on the amount of free space available on each OST and less on how stripes are distributed across OSTs. The default value is 91 percent weighting for free space rebalancing and 9 percent for OST balancing. When the free space priority is set to 100, weighting is based entirely on free space and location is no longer used by the striping algorithm.
• Introduced in Lustre 2.9osp.*.reserved_mb_low - The low watermark used to stop object allocation if available space is less than this. The default is 0.1% of total OST size.
• Introduced in Lustre 2.9osp.*.reserved_mb_high - The high watermark used to start object allocation if available space is more than this. The default is 0.2% of total OST size.

For more information about monitoring and managing free space, see the section called “Managing Free Space”.

Configuring Locking

The lru_size parameter is used to control the number of client-side locks in the LRU cached locks queue. LRU size is normally dynamic, based on load to optimize the number of locks cached on nodes that have different workloads (e.g., login/build nodes vs. compute nodes vs. backup nodes).

The total number of locks available is a function of the server RAM. The default limit is 50 locks/1 MB of RAM. If memory pressure is too high, the LRU size is shrunk. The number of locks on the server is limited tonum_osts_per_oss * num_clients * lru_size as follows:

• To enable automatic LRU sizing, set the lru_size parameter to 0. In this case, the lru_size parameter shows the current number of locks being used on the client. Dynamic LRU resizing is enabled by default.
• To specify a maximum number of locks, set the lru_size parameter to a value other than zero. A good default value for compute nodes is around 100 * *num_cpus*. It is recommended that you only set lru_size to be signifivantly larger on a few login nodes where multiple users access the file system interactively.

To clear the LRU on a single client, and, as a result, flush client cache without changing the lru_size value, run:

# lctl set_param ldlm.namespaces.osc_name|mdc_name.lru_size=clear

If the LRU size is set lower than the number of existing locks, unused locks are canceled immediately. Use clear to cancel all locks without changing the value.

Note

The lru_size parameter can only be set temporarily using lctl set_param, it cannot be set permanently.

To disable dynamic LRU resizing on the clients, run for example:

# lctl set_param ldlm.namespaces.*osc*.lru_size=5000

To determine the number of locks being granted with dynamic LRU resizing, run:

$ lctl get_param ldlm.namespaces.*.pool.limit

The lru_max_age parameter is used to control the age of client-side locks in the LRU cached locks queue. This limits how long unused locks are cached on the client, and avoids idle clients from holding locks for an excessive time, which reduces memory usage on both the client and server, as well as reducing work during server recovery.

The lru_max_age is set and printed in milliseconds, and by default is 3900000 ms (65 minutes).

Introduced in Lustre 2.11Since Lustre 2.11, in addition to setting the maximum lock age in milliseconds, it can also be set using a suffix of sor ms to indicate seconds or milliseconds, respectively. For example to set the client's maximum lock age to 15 minutes (900s) run:

# lctl set_param ldlm.namespaces.*MDT*.lru_max_age=900s
# lctl get_param ldlm.namespaces.*MDT*.lru_max_age
ldlm.namespaces.myth-MDT0000-mdc-ffff8804296c2800.lru_max_age=900000

Setting MDS and OSS Thread Counts

MDS and OSS thread counts tunable can be used to set the minimum and maximum thread counts or get the current number of running threads for the services listed in the table below.

Service	Description
mds.MDS.mdt	Main metadata operations service
mds.MDS.mdt_readpage	Metadata readdir service
mds.MDS.mdt_setattr	Metadata setattr/close operations service
ost.OSS.ost	Main data operations service
ost.OSS.ost_io	Bulk data I/O services
ost.OSS.ost_create	OST object pre-creation service
ldlm.services.ldlm_canceld	DLM lock cancel service
ldlm.services.ldlm_cbd	DLM lock grant service

For each service, tunable parameters as shown below are available.

• To temporarily set these tunables, run:

  # lctl set_param service.threads_min|max|started=num 
  

• To permanently set this tunable, run:

  # lctl conf_param obdname|fsname.obdtype.threads_min|max|started 
  

  Introduced in Lustre 2.5For version 2.5 or later, run:# lctl set_param -P *service*.threads_*min|max|started*

The following examples show how to set thread counts and get the number of running threads for the service ost_io using the tunable *service*.threads_*min|max|started*.

• To get the number of running threads, run:

  # lctl get_param ost.OSS.ost_io.threads_started
  ost.OSS.ost_io.threads_started=128
  

• To set the number of threads to the maximum value (512), run:

  # lctl get_param ost.OSS.ost_io.threads_max
  ost.OSS.ost_io.threads_max=512
  

• To set the maximum thread count to 256 instead of 512 (to avoid overloading the storage or for an array with requests), run:

  # lctl set_param ost.OSS.ost_io.threads_max=256
  ost.OSS.ost_io.threads_max=256
  

• To set the maximum thread count to 256 instead of 512 permanently, run:

  # lctl conf_param testfs.ost.ost_io.threads_max=256
  

  Introduced in Lustre 2.5For version 2.5 or later, run:# lctl set_param -P ost.OSS.ost_io.threads_max=256 ost.OSS.ost_io.threads_max=256

• To check if the threads_max setting is active, run:

  # lctl get_param ost.OSS.ost_io.threads_max
  ost.OSS.ost_io.threads_max=256
  

Note

If the number of service threads is changed while the file system is running, the change may not take effect until the file system is stopped and rest. If the number of service threads in use exceeds the new threads_max value setting, service threads that are already running will not be stopped.

See also Tuning a Lustre File System

Enabling and Interpreting Debugging Logs

By default, a detailed log of all operations is generated to aid in debugging. Flags that control debugging are found via lctl get_param debug.

The overhead of debugging can affect the performance of Lustre file system. Therefore, to minimize the impact on performance, the debug level can be lowered, which affects the amount of debugging information kept in the internal log buffer but does not alter the amount of information to goes into syslog. You can raise the debug level when you need to collect logs to debug problems.

The debugging mask can be set using "symbolic names". The symbolic format is shown in the examples below.

• To verify the debug level used, examine the parameter that controls debugging by running:

  # lctl get_param debug 
  debug=
  ioctl neterror warning error emerg ha config console
  

• To turn off debugging except for network error debugging, run the following command on all nodes concerned:

  # sysctl -w lnet.debug="neterror" 
  debug=neterror
  

• To turn off debugging completely (except for the minimum error reporting to the console), run the following command on all nodes concerned:

  # lctl set_param debug=0 
  debug=0
  

• To set an appropriate debug level for a production environment, run:

  # lctl set_param debug="warning dlmtrace error emerg ha rpctrace vfstrace" 
  debug=warning dlmtrace error emerg ha rpctrace vfstrace
  

  The flags shown in this example collect enough high-level information to aid debugging, but they do not cause any serious performance impact.

• To add new flags to flags that have already been set, precede each one with a "+":

  # lctl set_param debug="+neterror +ha" 
  debug=+neterror +ha
  # lctl get_param debug 
  debug=neterror warning error emerg ha console
  

• To remove individual flags, precede them with a "-":

  # lctl set_param debug="-ha" 
  debug=-ha
  # lctl get_param debug 
  debug=neterror warning error emerg console
  

Debugging parameters include:

• subsystem_debug - Controls the debug logs for subsystems.
• debug_path - Indicates the location where the debug log is dumped when triggered automatically or manually. The default path is /tmp/lustre-log.

These parameters can also be set using:

sysctl -w lnet.debug={value}

Additional useful parameters:

• panic_on_lbug - Causes ''panic'' to be called when the Lustre software detects an internal problem (an LBUGlog entry); panic crashes the node. This is particularly useful when a kernel crash dump utility is configured. The crash dump is triggered when the internal inconsistency is detected by the Lustre software.

• upcall - Allows you to specify the path to the binary which will be invoked when an LBUG log entry is encountered. This binary is called with four parameters:

  - The string ''LBUG''.

  - The file where the LBUG occurred.

  - The function name.

  - The line number in the file

Interpreting OST Statistics

Note

See also the section called “ llobdstat”(llobdstat) and the section called “ CollectL ” (collectl).

OST stats files can be used to provide statistics showing activity for each OST. For example:

# lctl get_param osc.testfs-OST0000-osc.stats 
snapshot_time                      1189732762.835363
ost_create                 1
ost_get_info               1
ost_connect                1
ost_set_info               1
obd_ping                   212

Use the llstat utility to monitor statistics over time.

To clear the statistics, use the -c option to llstat. To specify how frequently the statistics should be reported (in seconds), use the -i option. In the example below, the -c option clears the statistics and -i10 option reports statistics every 10 seconds:

$ llstat -c -i10 ost_io
 
/usr/bin/llstat: STATS on 06/06/07 
        /proc/fs/lustre/ost/OSS/ost_io/ stats on 192.168.16.35@tcp
snapshot_time                              1181074093.276072
 
/proc/fs/lustre/ost/OSS/ost_io/stats @ 1181074103.284895
Name        Cur.  Cur. #
            Count Rate Events Unit  last   min    avg       max    stddev
req_waittime 8    0    8    [usec]  2078   34     259.75    868    317.49
req_qdepth   8    0    8    [reqs]  1      0      0.12      1      0.35
req_active   8    0    8    [reqs]  11     1      1.38      2      0.52
reqbuf_avail 8    0    8    [bufs]  511    63     63.88     64     0.35
ost_write    8    0    8    [bytes] 169767 72914  212209.62 387579 91874.29
 
/proc/fs/lustre/ost/OSS/ost_io/stats @ 1181074113.290180
Name        Cur.  Cur. #
            Count Rate Events Unit  last    min   avg       max    stddev
req_waittime 31   3    39   [usec]  30011   34    822.79    12245  2047.71
req_qdepth   31   3    39   [reqs]  0       0     0.03      1      0.16
req_active   31   3    39   [reqs]  58      1     1.77      3      0.74
reqbuf_avail 31   3    39   [bufs]  1977    63    63.79     64     0.41
ost_write    30   3    38   [bytes] 1028467 15019 315325.16 910694 197776.51
 
/proc/fs/lustre/ost/OSS/ost_io/stats @ 1181074123.325560
Name        Cur.  Cur. #
            Count Rate Events Unit  last    min    avg       max    stddev
req_waittime 21   2    60   [usec]  14970   34     784.32    12245  1878.66
req_qdepth   21   2    60   [reqs]  0       0      0.02      1      0.13
req_active   21   2    60   [reqs]  33      1      1.70      3      0.70
reqbuf_avail 21   2    60   [bufs]  1341    63     63.82     64     0.39
ost_write    21   2    59   [bytes] 7648424 15019  332725.08 910694 180397.87

The columns in this example are described in the table below.

Parameter	Description
Name	Name of the service event. See the tables below for descriptions of service events that are tracked.
Cur. Count	Number of events of each type sent in the last interval.
Cur. Rate	Number of events per second in the last interval.
# Events	Total number of such events since the events have been cleared.
Unit	Unit of measurement for that statistic (microseconds, requests, buffers).
last	Average rate of these events (in units/event) for the last interval during which they arrived. For instance, in the above mentioned case of ost_destroy it took an average of 736 microseconds per destroy for the 400 object destroys in the previous 10 seconds.
min	Minimum rate (in units/events) since the service started.
avg	Average rate.
max	Maximum rate.
stddev	Standard deviation (not measured in some cases)

Events common to all services are shown in the table below.

Parameter	Description
req_waittime	Amount of time a request waited in the queue before being handled by an available server thread.
req_qdepth	Number of requests waiting to be handled in the queue for this service.
req_active	Number of requests currently being handled.
reqbuf_avail	Number of unsolicited lnet request buffers for this service.

Some service-specific events of interest are described in the table below.

Parameter	Description
ldlm_enqueue	Time it takes to enqueue a lock (this includes file open on the MDS)
mds_reint	Time it takes to process an MDS modification record (includes create, mkdir, unlink, renameand setattr)

Interpreting MDT Statistics

Note

See also the section called “ llobdstat”(llobdstat) and the section called “ CollectL ” (collectl).

MDT stats files can be used to track MDT statistics for the MDS. The example below shows sample output from an MDT stats file.

# lctl get_param mds.*-MDT0000.stats
snapshot_time                   1244832003.676892 secs.usecs 
open                            2 samples [reqs]
close                           1 samples [reqs]
getxattr                        3 samples [reqs]
process_config                  1 samples [reqs]
connect                         2 samples [reqs]
disconnect                      2 samples [reqs]
statfs                          3 samples [reqs]
setattr                         1 samples [reqs]
getattr                         3 samples [reqs]
llog_init                       6 samples [reqs] 
notify                          16 samples [reqs]