linux-kernel - Re: SATA RAID5 speed drop of 100 MB/s

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Message-ID: <467E9356.1030200@msgid.tls.msk.ru>
Date:	Sun, 24 Jun 2007 19:52:54 +0400
From:	Michael Tokarev <mjt@....msk.ru>
To:	Justin Piszcz <jpiszcz@...idpixels.com>
CC:	"Dr. David Alan Gilbert" <linux@...blig.org>,
	Jeff Garzik <jeff@...zik.org>, Carlo Wood <carlo@...noe.com>,
	Tejun Heo <htejun@...il.com>,
	Manoj Kasichainula <manoj@...com>,
	linux-kernel@...r.kernel.org,
	IDE/ATA development list <linux-ide@...r.kernel.org>
Subject: Re: SATA RAID5 speed drop of 100 MB/s

Justin Piszcz wrote:
> Don't forget about max_sectors_kb either (for all drives in the SW RAID5
> array)
> 
> max_sectors_kb = 8
> $ dd if=/dev/zero of=file.out6 bs=1M count=10240
> 10737418240 bytes (11 GB) copied, 55.4848 seconds, 194 MB/s
> 
> max_sectors_kb = 128
> 10737418240 bytes (11 GB) copied, 22.6298 seconds, 474 MB/s

Well.  You're comparing something different.  Yes, this
thread is about linux software raid5 in the first place,
but I were commenting about [NT]CQ within a single drive.

Overall, yes, the larger your reads/writes to the drive
becomes, the faster its linear performance is.  Yet you
have to consider real workload instead of very synthetic
dd test.  It may be good approcsimation of a streaming
video workload (when you feed a large video file over
network or something like that), but even with this,
you probably want to feed several files at once (different
files to different clients), so single-threaded test
here isn't very useful.  IMHO anyway, and good for a
personal computer test.

/mjt
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