[LU-2716] DNE on ZFS create remote directory suffers from long sync.

• Type: Bug
• Resolution: Unresolved
• Priority: Major
• Affects Version/s: Lustre 2.4.0
• Labels: dne performance zfs
• Assignee: WC Triage
• Reporter: Di Wang

Description

The ZFS transaction sync operation will passively wait for a txg commit, instead of actively requesting TXG commit start before waiting. This causes each synchronous operation to wait up to 5s for the normal txg flush.

Until such a time that we have implemented Lustre ZIL support (LU-4009), it makes sense that transaction handles that are marked as synchronous start a TXG commit instead of passively waiting. This might impact aggregate performance, but DNE operations will be relatively rare, and should not impact normal operations noticeably. That is especially true for flash-based ZFS devices, since the IOPS to update the überblock copies at the start of each block device do not need multiple mechanical full-patter seeks, so the cost of forcing a commit is relatively low.

As an optimization, there could be a "batch wait and reschedule" for TXG sync, as there is for jbd2 since commit v2.6.28-5737-ge07f7183a486 "jbd2: improve jbd2 fsync batching" so that it allows multiple active threads to join the same TXG before it is closed for commit but does not wait unnecessarily between commits.

Issue Links

• is related to: LU-4009 Add ZIL support to osd-zfs