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

[Zdenek Kabelac]

Dne 11.7.2016 v 22:44 Jon Bernard napsal(a):
> 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.


Have you tried different 'chunk-sizes' ?

The smaller the chunk/block-size  is - the better snapshot utilization is,
but more contention (e.g. try 512K)

Also there is a big difference when you perform initial block provisioning
or you use already provisioned block -  so the 'more' realistic measurement
should be taken on already provisioned thin device.

And finally -  thin devices from a single thin-pool are not meant to be 
heavily used in parallel (I'd not recommend to use more then 16 devs) - there 
is still large room for improvement, but correctness has the priority.

Regards

Zdenek