[2.1.10] ZFS_IOC_COUNT_FILLED does unnecessary txg_wait_synced() #14627
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`lseek(SEEK_DATA | SEEK_HOLE)` are only accurate when the on-disk blocks reflect all writes, i.e. when there are no dirty data blocks. To ensure this, if the target dnode is dirty, they wait for the open txg to be synced, so we can call them "stabilizing operations". If they cause txg_wait_synced often, it can be detrimental to performance. Typically, a group of files are all modified, and then SEEK_DATA/HOLE are performed on them. In this case, the first SEEK does a txg_wait_synced(), and subsequent SEEKs don't need to wait, so performance is good. However, if a workload involves an interleaved metadata modification, the subsequent SEEK may do a txg_wait_synced() unnecessarily. For example, if we do a `read()` syscall to each file before we do its SEEK. This applies even with `relatime=on`, when the `read()` is the first read after the last write. The txg_wait_synced() is unnecessary because the SEEK operations only care that the structure of the tree of indirect and data blocks is up to date on disk. They don't care about metadata like the contents of the bonus or spill blocks. (They also don't care if an existing data block is modified, but this would be more involved to filter out.) This commit changes the behavior of SEEK_DATA/HOLE operations such that they do not call txg_wait_synced() if there is only a pending change to the bonus or spill block. Reviewed-by: Brian Behlendorf <[email protected]> Reviewed-by: Alexander Motin <[email protected]> Signed-off-by: Matthew Ahrens <[email protected]> Closes openzfs#13368 Issue openzfs#14594 Issue openzfs#14512 Issue openzfs#14009
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Motivation and Context
Backport for 2.1.10.
Closes #13368
Issue #14594
Issue #14512
Issue #14009
Description
lseek(SEEK_DATA | SEEK_HOLE)are only accurate when the on-disk blocks reflect all writes, i.e. when there are no dirty data blocks. To ensure this, if the target dnode is dirty, they wait for the open txg to be synced, so we can call them "stabilizing operations". If they cause txg_wait_synced often, it can be detrimental to performance.Typically, a group of files are all modified, and then SEEK_DATA/HOLE are performed on them. In this case, the first SEEK does a txg_wait_synced(), and subsequent SEEKs don't need to wait, so performance is good.
However, if a workload involves an interleaved metadata modification, the subsequent SEEK may do a txg_wait_synced() unnecessarily. For example, if we do a
read()syscall to each file before we do its SEEK. This applies even withrelatime=on, when theread()is the first read after the last write. The txg_wait_synced() is unnecessary because the SEEK operations only care that the structure of the tree of indirect and data blocks is up to date on disk. They don't care about metadata like the contents of the bonus or spill blocks. (They also don't care if an existing data block is modified, but this would be more involved to filter out.)This commit changes the behavior of SEEK_DATA/HOLE operations such that they do not call txg_wait_synced() if there is only a pending change to the bonus or spill block.
Types of changes