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

[Jack Wang]

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?

Regards,
Jack

>
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