linux-kernel - Re: Linux I/O subsystem performance

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Date:	Wed, 25 Aug 2010 23:12:55 +0400
From:	Vladislav Bolkhovitin <vst@...b.net>
To:	Chris Worley <worleys@...il.com>
CC:	Pasi Kärkkäinen <pasik@....fi>,
	Chetan Loke <chetanloke@...il.com>,
	Bart Van Assche <bvanassche@....org>,
	linux-scsi@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	James Bottomley <James.Bottomley@...e.de>,
	scst-devel <scst-devel@...ts.sourceforge.net>
Subject: Re: Linux I/O subsystem performance

Chris Worley, on 08/25/2010 12:31 AM wrote:
>> I also have an impression that Linux I/O subsystem has some performance
>> problems. For instance, in one recent SCST performance test only 8 Linux
>> initiators with fio as a load generator were able to saturate a single SCST
>> target with dual IB cards (SRP) on 4K AIO direct accesses over an SSD
>> backend. This rawly means that any initiator took several times (8?) more
>> processing time than the target.
>
> While I can't tell you where the bottlenecks are, I can share some
> performance numbers...
>
> 4 initiators can get>600K random 4KB IOPS off a single target...

Hmm, on the data you sent me only 8 initiators were capable to do so... 
I'm glad to see an improvement here ;).

> which is ~150% of what the Emulex/Intel/Microsoft results show using 8
> targets at 4KB (their 1M IOPS was at 512 byte blocks, which is not a
> realistic test point

 From my, a storage developer's, POV it isn't about if this test is 
realistic or not. 512 bytes tests are good if you want to test how 
processing effective your I/O stack, because they produce the max 
possible CPU/memory/hardware interaction load. Since processing power 
isn't unlimited, in case if it is a bottleneck, N IOPS on 512b < N IOPS 
on 4K * 8 and system with more effective processing will have better 
numbers.

Vlad
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