linux-kernel - Re: Performance Characteristics of All Linux RAIDs (mdadm/bonnie++)

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Message-ID: <483DE40D.8090608@tmr.com>
Date:	Wed, 28 May 2008 19:00:29 -0400
From:	Bill Davidsen <davidsen@....com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Jens Bäckman <jens.backman@...il.com>,
	Justin Piszcz <jpiszcz@...idpixels.com>,
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
	xfs@....sgi.com
Subject: Re: Performance Characteristics of All Linux RAIDs (mdadm/bonnie++)

Alan Cox wrote:
> On Wed, 28 May 2008 18:34:00 +0200
> "Jens Bäckman" <jens.backman@...il.com> wrote:
> 
>> On Wed, May 28, 2008 at 10:53 AM, Justin Piszcz <jpiszcz@...idpixels.com> wrote:
>>> Results:
>>>
>>> http://home.comcast.net/~jpiszcz/raid/20080528/raid-levels.html
>>> http://home.comcast.net/~jpiszcz/raid/20080528/raid-levels.txt
>> Either the RAID 1 read speed must be wrong, or something is odd in the
>> Linux implementation. There's six drives that can be used for reading
>> at the same time, as they contain the very same data. 63MB/s
>> sequential looks like what you would get from a single drive.
> 
> Which is fairly typical of a cheap desktop PC where the limitation is the
> memory and PCI bridge as much as the drive.
> 
I really don't think that's any part of the issue, the same memory and 
bridge went 4-5x faster in other read cases. The truth is that the 
raid-1 performance is really bad, and it's the code causing it AFAIK. If 
you track the actual io it seems to read one drive at a time, in order, 
without overlap.

-- 
Bill Davidsen <davidsen@....com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

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