linux-kernel - Re: Slow file transfer speeds with CFQ IO scheduler in some cases

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Date:	Fri, 20 Mar 2009 11:53:34 +0300
From:	Vladislav Bolkhovitin <vst@...b.net>
To:	Wu Fengguang <wfg@...ux.intel.com>
CC:	Jens Axboe <jens.axboe@...cle.com>, Jeff Moyer <jmoyer@...hat.com>,
	"Vitaly V. Bursov" <vitalyb@...enet.dn.ua>,
	linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org,
	lukasz.jurewicz@...il.com
Subject: Re: Slow file transfer speeds with CFQ IO scheduler in some cases

Vladislav Bolkhovitin, on 03/19/2009 08:44 PM wrote:
>   6. Unexpected result. In case, when all IO threads work in the same IO 
> context with CFQ increasing RA size *decreases* throughput. I think this 
> is, because RA requests performed as single big READ requests, while 
> requests coming from remote clients are much smaller in size (up to 
> 256K), so, when the read by RA data transferred to the remote client on 
> 100MB/s speed, the backstorage media gets rotated a bit, so the next 
> read request must wait the rotation latency (~0.1ms on 7200RPM).
                                                ^^^^^^^^^^^^^^^^^
Oops, it's RP*M*, hence the full rotation latency is in 60 times more, 
i.e. 8.3 ms.

Sorry,
Vlad

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