[LU-14712] improve ldiskfs "-o discard" performance

• Type: Improvement
• Resolution: Unresolved
• Priority: Minor
• Labels: NVME ldiskfs performance
• Assignee: Dongyang Li
• Reporter: Andreas Dilger

Description

The current "-o discard" mount option for ldiskfs enables on-the-fly TRIM of underlying flash devices (or thinly-provisioned LUNs). However, the current implementation hurts performance because it is done synchronously in the context of the JBD2 commit thread as a commit callback, which blocks later transaction commits.

There is work being done in the upstream kernel to improve "-o discard" to use an async worker thread to issue the TRIM commands, and do this on a per-blockgroup basis, rather than issuing the trim on a per-extent basis.

Current patch series is at:
https://marc.info/?l=linux-ext4&m=162201857620045&w=4

There are also patches by Shilong that make the TRIM state for a block group persistent, so that running TRIM with mke2fs does not also lead to fstrim resubmitting TRIM requests for all of the groups again immediately after mount/remount.

Most recent patches at:
https://marc.info/?l=linux-ext4&m=159283169109297&w=4

I think the combination of these two patches would improve ongoing ldiskfs performance on flash devices significantly.

Issue Links

• is related to: LU-16750 optimize ldiskfs internal metadata allocation for hybrid storage LUNs