On Tue, Jul 12 2016 at 11:29pm -0400, Jon Bernard <jbernard@xxxxxxxxxx> wrote: > * Jack Wang <jack.wang.usish@xxxxxxxxx> wrote: > > 2016-07-11 22:44 GMT+02:00 Jon Bernard <jbernard@xxxxxxxxxx>: > > > Greetings, > > > > > > I have recently noticed a large difference in performance between thick > > > and thin LVM volumes and I'm trying to understand why that it the case. > > > > > > In summary, for the same FIO test (attached), I'm seeing 560k iops on a > > > thick volume vs. 200k iops for a thin volume and these results are > > > pretty consistent across different runs. > > > > > > I noticed that if I run two FIO tests simultaneously on 2 separate thin > > > pools, I net nearly double the performance of a single pool. And two > > > tests on thin volumes within the same pool will split the maximum iops > > > of the single pool (essentially half). And I see similar results from > > > linux 3.10 and 4.6. > > > > > > I understand that thin must track metadata as part of its design and so > > > some additional overhead is to be expected, but I'm wondering if we can > > > narrow the gap a bit. > > > > > > In case it helps, I also enabled LOCK_STAT and gathered locking > > > statistics for both thick and thin runs (attached). > > > > > > I'm curious to know whether this is a know issue, and if I can do > > > anything the help improve the situation. I wonder if the use of the > > > primary spinlock in the pool structure could be improved - the lock > > > statistics appear to indicate a significant amount of time contending > > > with that one. Or maybe it's something else entirely, and in that case > > > please enlighten me. > > > > > > If there are any specific questions or tests I can run, I'm happy to do > > > so. Let me know how I can help. > > > > > > -- > > > Jon > > > > Hi Jon, > > > > Have you try to enable scsi_mq mode in newer kernel eg 4.6, see if it > > makes any difference? > > Thanks for the suggestion, I had not tried it previously. I added > 'scsi_mod.usb_blk_mq=Y' and 'dm_mod.use_blk_mq=Y' to my kernel command > line and verified the mq subdirectory contents in /sys/block/<device>. > All seemed to be correctly enabled. I also realized that > dm_mod.use_blk_mq is only for multipath, so I don't think it's relevant > to my tests. Yes dm_mod.use_blk_mq is specific to request-based DM. But using scsi-mq will eliminate any q->queue_lock contention from the underlying SCSI device that you have in your current lockstat. > Results were very similar to previous tests, ~10x slowdown from thick to > thin. Mike raised several good points, I'm re-running the tests and > will post new results in response. OK, thanks. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
