Add SSD Health feature design

doc/ssdhealth_design.md

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+## Motivation
+Add to SONiC an ability to check storage health state. Basic functionality will be implemented as a CLI command. Optionally pmon daemon could be added for constant disk state monitoring.
+
+## CLI
+
+### Syntax
+    show platform ssdhealth [verbose/vendor]
+
+### Output example
+#### Brief
+    admin@sonic-switch: ~$ show platform ssdhealth
+    Device Model : InnoDisk Corp. - mSATA 3ME
+    Health: 72.9%
+    Temperature: N/A
+    admin@sonic-switch: ~$
+
+#### Verbose
+    admin@sonic-switch: ~$ show platform ssdhealth verbose
+    Device Model     : InnoDisk Corp. - mSATA 3ME
+    FW Version       : S140714
+    Serial Number    : 20160429AA1134000035
+    Health           : 72.9%
+    Capacity         : 29.818199 GB
+    Temperature      : N/A
+    Power On Hours   : 1576 hours
+    Power Cycle count: 130
+    Something else???
+
+#### Vendor
+    admin@sonic-switch: ~$ show platform ssdhealth vendor
+
+	********************************************************************************************
+	* Innodisk iSMART V3.9.41                                                       2018/05/25 *
+	********************************************************************************************
+	Model Name: InnoDisk Corp. - mSATA 3ME
+	FW Version: S140714
+	Serial Number: 20160429AA1134000035
+	Health: 72.900%
+	Capacity: 29.818199 GB
+	P/E Cycle: 3000
+	Lifespan : 1576 (Years : 4 Months : 3 Days : 26)
+	Write Protect: Disable
+	InnoRobust: Enable
+	--------------------------------------------------------------------------------------------
+	ID    SMART Attributes                            Value           Raw Value
+	--------------------------------------------------------------------------------------------
+	[09]  Power On Hours                              [18304]         [090200646480470000000000]
+	[0C]  Power Cycle Count                           [  130]         [0C0200646482000000000000]
+	[AA]  Total Bad Block Count                       [   15]         [AA0300646400000F00000000]
+	[AD]  Erase Count Max.                            [  883]         [AD020064642D037303000000]
+	[AD]  Erase Count Avg.                            [  813]         [AD020064642D037303000000]
+	[C2]  Temperature                                 [    0]         [000000000000000000000000]
+	[EB]  Later Bad Block                             [    0]         [EB0200640000000000000000]
+	[EB]  Read Block                                  [    0]         [EB0200640000000000000000]
+	[EB]  Write Block                                 [    0]         [EB0200640000000000000000]
+	[EB]  Erase Block                                 [    0]         [EB0200640000000000000000]
+	[EC]  Unstable Power Count                        [    0]         [EC0200646400000000000000]
+	admin@sonic-switch: ~$
+
+## Implementation
+### Generic part
+#### 'show' utility update
+New item under menu `platform` in `show/main.py`  
+It will execute "ssdhealth -d /dev/sdX" [options]
+
+#### ssdhealth utility
+New utility in `sonic-utilities/scripts/`  
+It will import device plugin `ssdutil.py` and print the output returned by different API functions  
+
+**Syntax:**
+
+	root@mts-sonic-dut:/home/admin# ssdhealth -h
+	usage: ssdhealth -d DEVICE [-h] [-v] [-e]
+
+	Show disk device health status
+
+	optional arguments:
+	  -h, --help            show this help message and exit
+	  -d, --device          disk device to get information for
+	  -v, --verbose         show verbose output (more parameters)
+	  -e, --vEndor          show vendor specific disk information
+
+	Examples:
+	  ssdhealth -d /dev/sda
+	  ssdhealth -d /dev/sda -v
+	  ssdhealth -d /dev/sda -e
+
+
+#### Plugins design
+##### Class SsdBase
+Location: `sonic-buildimage/src/sonic-platform-common/sonic_platform_base/sonic_ssd/ssd_base.py`  
+Generic implementation of the API. Will use specific utilities for known disks or the `systemctl` utility for others. Since not all disk models are in smartctl's database, some information can be unavailable or incomplete.
+
+    class SsdBase:
+      ...
+
+##### Class SsdUtil
+Inherited from SsdBase. Can be implemented by vendors to provide detailed info about the disk installed.  
+Location: `sonic-buildimage/device/{{vendor}}/platform/plugins/ssdutil.py`  
+
+    class SsdUtil(SsdBase):
+      ...
+
+#### API
+* **get\_disk\_health(diskdev)**
+	* Accepts:
+		* diskdev:string  - disk device name (e.g. /dev/sda)
+	* Returns:
+		* res:float - Floating point in range 0-100 representing disk health in percentages. -1 if not available
+* **get\_temperature(diskdev)**
+	* Accepts:
+		* diskdev:string  - disk device name (e.g. /dev/sda)
+	* Returns:
+		* res:string - Integer (floating point?) disk temperature in centigrade. Zero if not available
+* **get\_model(diskdev)**
+	* Accepts:
+		* diskdev:string  - disk device name (e.g. /dev/sda)
+	* Returns:
+		* res:string - Human readable string holding disk model. Empty if not available
+* **get\_firmware(diskdev)**
+	* Accepts:
+		* diskdev:string  - disk device name (e.g. /dev/sda)
+	* Returns:
+		* res:string - Human readable string holding disk firmware version. Empty if not available
+* **get\_serial(diskdev)**
+	* Accepts:
+		* diskdev:string  - disk device name (e.g. /dev/sda)
+	* Returns:
+		* res:string - Human readable string holding disk serial number. Empty if not available
+* **get\_vendor_output(diskdev)**
+	* Accepts:
+		* diskdev:string  - disk device name (e.g. /dev/sda)
+	* Returns:
+		* res:string - Human readable string. Output of vendor application. Empty if not available
+
+## Utilities and packages
+#### smartctl
+Part of smartmontools package (1.9M)  
+PR: [https://github.com/Azure/sonic-buildimage/pull/2703](https://github.com/Azure/sonic-buildimage/pull/2703)
+
+#### iSmart
+Utility for InnoDisk Corp. SSDs (<120K)
+https://www.innodisk.com/en/iService/utility/iSMART  
+Need to be added as binary.
+
+#### SmartCmd
+Utility for StorFly and Virtium (2.2M)  
+
+## (Optional) Daemon for monitoring
+Daemon in Pmon (ssdmond) which will periodically query disk health (get_health()) and raise alarm when value decides  to some critical value.  
+
+## Open questions
+1. Daemon and monitoring?
+2. SNMP needed?
+