ZFS send should use spill block prefetched from send_reader_thread

[tuxoko]

Motivation and Context

Currently, even though send_reader_thread prefetches spill block, do_dump() will not use it and issues its own blocking arc_read. This may cause significant performance degradation when sending datasets with lots of spill blocks.

For unmodified spill blocks, we also create send_range struct for them in send_reader_thread and issue prefetches for them. We piggyback them on the dnode send_range instead of enqueueing them so we don't break send_range_after check.

On one of our setup, we see at least 5x difference during affected areas.

Description

How Has This Been Tested?

Manual send/recv on dataset with spill block.
Manual raw send/recv on encrypted dataset with spill block.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[amotin]

Seems to make sense.

@ahrens do think it was just missed from #10067, or there is something behind it?

[tuxoko]

Not sure why tests are showing failure even though the test results seems mostly fine.
Rebase and rerun the tests.

[amotin]

Crashed on assertion:

  [ 5544.971043] VERIFY(dscp->dsc_dso->dso_dryrun || srdp->abuf != NULL || srdp->abd != NULL) failed
  [ 5544.974720] PANIC at dmu_send.c:980:do_dump()

[tuxoko]

Crashed on assertion:

  [ 5544.971043] VERIFY(dscp->dsc_dso->dso_dryrun || srdp->abuf != NULL || srdp->abd != NULL) failed
  [ 5544.974720] PANIC at dmu_send.c:980:do_dump()

Hmm...
This doesn't show up in our internal CI for some reason.

Edit: Ok, I think it's because we turn off zfs_send_unmodified_spill_blocks

[tuxoko]

Updated to prefetch also unmodified spill block in send_reader_thread.