From: NeilBrown <neilb@suse.de>
To: Nikolay Kichukov <hijacker@oldum.net>
Cc: Roman Mamedov <rm@romanrm.ru>,
Liam Kurmos <quantum.leaf@gmail.com>,
Roberto Spadim <roberto@spadim.com.br>,
Brad Campbell <lists2009@fnarfbargle.com>,
Drew <drew.kay@gmail.com>,
linux-raid@vger.kernel.org
Subject: Re: mdadm raid1 read performance
Date: Thu, 5 May 2011 19:30:39 +1000 [thread overview]
Message-ID: <20110505193039.7c75dd80@notabene.brown> (raw)
In-Reply-To: <4DC25A71.6090705@oldum.net>
On Thu, 05 May 2011 11:06:09 +0300 Nikolay Kichukov <hijacker@oldum.net>
wrote:
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>
> Hi,
> It seems like he is reading directly from the raid device and not through the filesystem. So there are no filesystem
> caches in this way.
A block device is actually implemented a lot like a filesystem which
contains a single file with a trivial mapping from file-block to device-block.
In any case, reading from a block device most definitely does go through the
page cache, and repeatedly reading from a device which is substantially
smaller than memory will cause subsequent reads to come from the cache.
NeilBrown
>
> Cheers,
> - -Nik
>
> On 05/05/2011 07:06 AM, Roman Mamedov wrote:
> > On Thu, 5 May 2011 00:08:59 +0100
> > Liam Kurmos <quantum.leaf@gmail.com> wrote:
> >
> >> in my tests i read 1GB and throw away the data.
> >> dd if=/dev/md0 of=/dev/null bs=1M count=1000
> >
> > If you have enough RAM for disk cache, on the second and further consecutive
> > invocations of this you will be reading mostly from the cache, giving you an
> > incorrect inflated result. So either don't forget to drop filesystem caches
> > between runs, or just test read performance with "hdparm -t /dev/mdX" instead.
> >
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next prev parent reply other threads:[~2011-05-05 9:30 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-05-04 0:07 mdadm raid1 read performance Liam Kurmos
2011-05-04 0:57 ` John Robinson
2011-05-06 20:44 ` Leslie Rhorer
2011-05-06 21:56 ` Keld Jørn Simonsen
2011-05-04 0:58 ` NeilBrown
2011-05-04 5:30 ` Drew
2011-05-04 6:31 ` Brad Campbell
2011-05-04 7:42 ` Roberto Spadim
2011-05-04 23:08 ` Liam Kurmos
2011-05-04 23:35 ` Roberto Spadim
2011-05-04 23:36 ` Brad Campbell
2011-05-04 23:45 ` NeilBrown
2011-05-04 23:57 ` Roberto Spadim
2011-05-05 0:14 ` Liam Kurmos
2011-05-05 0:20 ` Liam Kurmos
2011-05-05 0:25 ` Roberto Spadim
2011-05-05 0:40 ` Liam Kurmos
2011-05-05 7:26 ` David Brown
2011-05-05 10:41 ` Keld Jørn Simonsen
2011-05-05 11:38 ` David Brown
2011-05-06 4:14 ` CoolCold
2011-05-06 7:29 ` David Brown
2011-05-06 21:05 ` Leslie Rhorer
2011-05-07 10:37 ` David Brown
2011-05-07 10:58 ` Keld Jørn Simonsen
2011-05-05 0:24 ` Roberto Spadim
2011-05-05 11:10 ` Keld Jørn Simonsen
2011-05-06 21:20 ` Leslie Rhorer
2011-05-06 21:53 ` Keld Jørn Simonsen
2011-05-07 3:17 ` Leslie Rhorer
2011-05-05 4:06 ` Roman Mamedov
2011-05-05 8:06 ` Nikolay Kichukov
2011-05-05 8:39 ` Liam Kurmos
2011-05-05 8:49 ` Liam Kurmos
2011-05-05 9:30 ` NeilBrown [this message]
2011-05-04 7:48 ` David Brown
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