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

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Date:	Wed, 26 Mar 2008 08:15:15 +0100
From:	"Bart Van Assche" <bart.vanassche@...il.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

On Tue, Mar 25, 2008 at 11:00 PM, Chris Snook <csnook@...hat.com> 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.

If you want to benchmark write speed, you should add
oflag=direct,dsync to the dd command line. For benchmarking read speed
you should specify iflag=direct. Or, even better, you can use xdd with
the flags -dio -processlock.

Bart.
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