Details
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Improvement
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Resolution: Fixed
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Minor
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None
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9223372036854775807
Description
Unlanded patches exist in upstream ZFS to increase the dnode size which need to be evaluated for their impact (hopefully improvement) for Lustre metadata performance on ZFS MDTs:
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- Resolved
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LU-8424 osd_object.c:1330:22: error: 'DN_MAX_BONUSLEN' undeclared (first use in this function) DMU_OT_SA, DN_MAX_BONUSLEN, tx);
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- Resolved
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- Resolved
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Activity
We're currently testing with the following patch to mitigate the performance impact of metadnode backfilling. It uses a naive heuristic (rescan after 4096 unlinks at most once per txg) but this is simple and probably achieves 99% of the performance to be gained here.
The large dnode patch is blocked behind https://github.com/zfsonlinux/zfs/pull/4460 which is the performance problem that Alex and Ned identified, but currently that patch is only a workaround and needs to be improved before landing. I've described in that ticket what seems to be a reasonable approach for making a production-ready solution, but to summarize:
- by default the dnode allocator should just use a counter that continues at the next file offset (as in the existing 4460 patch)
- if dnodes are being unlinked, a (per-cpu?) counter of unlinked dnodes and the minimum unlinked dnode number should be tracked (these values could be racy since it isn't critical that their values be 100% accurate)
- when the unlinked dnode counter exceeds some threshold (e.g. 4x number of inodes created in previous TXG, or 64x the number of dnodes that fit into a leaf block, or some tunable number of unlinked dnodes specified by userspace) then scanning should restart at the minimum unlinked dnode number instead of "0" to avoid scanning a large number of already-allocated dnode blocks
Alex, in order to move the large dnode patch forward, could you or Nathaniel work on an updated 4460 patch so that we can get on with landing the large dnode patch.
Ned refreshed the patch to address that performance issue and now it's doing much better.
first of all, now I'm able to complete some tests where I was getting OOM (because of huge memory consumption by 8K spill, I guess).
now it makes sense to benchmark on a real storage as amount of IO with this patch is few times less:
1K vs (512byte dnode + 8K spill) per dnode OR 976MB vs 8300MB per 1M dnodes.
tried Lustre with that patch (on top of master ZFS):
before:
mdt 1 file 500000 dir 1 thr 1 create 21162.48 [ 18998.73, 22999.10]
after:
mdt 1 file 500000 dir 1 thr 1 create 18019.70 [ 15999.09, 19999.20]
osd-zfs/ was modified to ask for 1K dnodes, verified with zdb:
Object lvl iblk dblk dsize dnsize lsize %full type
10000 1 16K 512 0 1K 512 0.00 ZFS plain file
notice zero dsize meaning no spill was allocated.
clean zfs/master:
Created 1000000 in 29414ms in 1 threads - 33997/sec
Created 1000000 in 20045ms in 2 threads - 49887/sec
Created 1000000 in 19259ms in 4 threads - 51923/sec
Created 1000000 in 17284ms in 8 threads - 57856/sec
zfs/master + large dnodes:
Created 1000000 in 40618ms in 1 threads - 24619/sec
Created 1000000 in 28142ms in 2 threads - 35534/sec
Created 1000000 in 25731ms in 4 threads - 38863/sec
Created 1000000 in 25244ms in 8 threads - 39613/sec
hmm, I was using old version.. let me try the new one. this will take some time - the patch doesn't apply to 0.6.5
yes, I was about to play with the code, but got confused by that performance issue. and yes, 1K should be more than enough: LinkEA would be 48+ bytes, LOVEA is something like 56+, then LMA and VBR (which I'd hope we can put into ZPL dnode, but in the worst case it's another 24+8 bytes).
Alex, your comment is on an old version of the patch, and not on the main pull request (https://github.com/zfsonlinux/zfs/pull/3542), so I don't think Ned will be looking there? Also, hopefully you are not using this old version of the patch (8f9fdb228), but rather the newest patch (ba39766)?
Rereading the large dnode patch, it seems that the caller can specify the dnode size on a per-dnode basis, so ideally we can add support for this to the osd-zfs code, but if not specified it will take the dataset property. Is 1KB large enough to hold the dnode + LOVEA + linkea + FID?
Andreas, I've made a comment to github already, no reply so far. Hope Ned has that seen.
so far I've tested large dnodes with ZPL only and noticed significant degradation, so I took a timeout hoping to see comments from Ned.
I haven't tested Lustre with large dnodes.
the patch allows to ask for dnode of specific size and I think we can do this given that we declare everything (including LOVEA of known size) ahead.
we can easly track this in OSD.
Ned Bass (bass6@llnl.gov) uploaded a new patch: http://review.whamcloud.com/20367
Subject:
LU-8068osd-zfs: large dnode compatibilityProject: fs/lustre-release
Branch: master
Current Patch Set: 1
Commit: f0d8afaec213a7f471c3b22b9940de5c5cd192e3