[stgt] poor rbd performance

Wyllys Ingersoll wyllys.ingersoll at keepertech.com
Mon Aug 25 05:38:10 CEST 2014

On Sun, Aug 24, 2014 at 7:25 PM, FUJITA Tomonori
<fujita.tomonori at lab.ntt.co.jp> wrote:
> Hello,
> On Sat, 23 Aug 2014 09:46:37 -0400
> Wyllys Ingersoll <wyllys.ingersoll at keepertech.com> wrote:
>> Hey Dan, its always good to hear from another former Sun eng.
>> I saw your blog posts about the RBD backend and it seems to work as
>> advertised.  I don't know if the rados aio calls will make it better
>> or not.  I do know that it is possible to get better throughput using
>> the standard IO functions (the ones you used) because all of the
>> non-iSCSI tests we've done just use the standard rbd_read/rbd_write
>> calls just like bs_rbd.  It might be an architectural limitation in
>> tgtd, but I am not familiar enough with the tgtd code yet to know
>> where to look for the bottlenecks.  I tried changing the timing
>> interval in 'work.c' to be much shorter, but it didn't make a
>> difference.  When I enabled some debug logging, I see that the maximum
>> data size that the tgtd read or write operation handles is only
>> 128Kb,
> You mean that the read/write size from iSCSI initiators is 128K? If
> so, probably it's due to your configuration. tgt doesn't have such
> limit. iSCSI has lots of negotiation parameters between an initiator
> and a target, which affects the performance. You need to configure
> both properly to get good performance.

I believe the initiator is from open-iSCSI.  I have tried changing
some settings but apparently haven't found the right ones that will
make a difference.

>> Any suggestions for further ceph tuning or other areas in tgtd to look
>> at for possible problems?
> I would suggest you to work on a simpler backend like bs_null to know
> the best performance of tgt on your env. I think that the first step
> is knowing where is the bottleneck.

Thanks, I'll take a look.
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