linux-kernel - Re: RAID-1 performance under 2.4 and 2.6

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Date:	Tue, 25 Mar 2008 18:47:36 -0400
From:	Bill Davidsen <davidsen@....com>
To:	Chris Snook <csnook@...hat.com>
CC:	Emmanuel Florac <eflorac@...ellique.com>,
	linux-kernel@...r.kernel.org
Subject: Re: RAID-1 performance under 2.4 and 2.6

Chris Snook wrote:
> Emmanuel Florac wrote:
>> I post there because I couldn't find any information about this
>> elsewhere : on the same hardware ( Athlon X2 3500+, 512MB RAM, 2x400 GB
>> Hitachi SATA2 hard drives ) the 2.4 Linux software RAID-1 (tested 2.4.32
>> and 2.4.36.2, slightly patched to recognize the hardware :p) is way
>> faster than 2.6 ( tested 2.6.17.13, 2.6.18.8, 2.6.22.16, 2.6.24.3)
>> especially for writes. I actually made the test on several different
>> machines (same hard drives though) and it remained consistent across
>> the board, with /mountpoint a software RAID-1.
>> Actually checking disk activity with iostat or vmstat shows clearly a
>> cache effect much more pronounced on 2.4 (i.e. writing goes on much
>> longer in the background) but it doesn't really account for the
>> difference. I've also tested it thru NFS from another machine (Giga
>> ethernet network):
>>
>> dd if=/dev/zero of=/mountpoint/testfile bs=1M count=1024
>>
>> kernel        2.4       2.6        2.4 thru NFS   2.6 thru NFS
>>
>> write        90 MB/s    65 MB/s      70 MB/s       45 MB/s
>> read         90 MB/s    80 MB/s      75 MB/s       65 MB/s
>>
>> Duh. That's terrible. Does it mean I should stick to  (heavily
>> patched...) 2.4 for my file servers or... ? :)
>>
> 
> It means you shouldn't use dd as a benchmark.
> 
What do you use as a benchmark for writing large sequential files or 
reading them, and why is it better than dd at modeling programs which 
read or write in a similar fashion?

Media programs often do data access in just this fashion, multi-channel 
video capture, streaming video servers, and similar.

-- 
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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