[dm-devel] poor thin performance, relative to thick

[Jon Bernard]

* Jack Wang <jack.wang.usish at gmail.com> wrote:
> 2016-07-11 22:44 GMT+02:00 Jon Bernard <jbernard at tuxion.com>:
> > 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.

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.

-- 
Jon