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Date: Mon, 30 Jan 2012 00:10:58 +0800
From: Wu Fengguang <wfg@linux.intel.com>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Herbert Poetzl <herbert@13thfloor.at>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, Jens Axboe <axboe@kernel.dk>,
        Tejun Heo <tj@kernel.org>
Subject: Re: Bad SSD performance with recent kernels
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On Sun, Jan 29, 2012 at 02:13:51PM +0100, Eric Dumazet wrote:
> Le dimanche 29 janvier 2012 à 19:16 +0800, Wu Fengguang a écrit :
> 
> 
> > Note that as long as buffered read(2) is used, it makes almost no
> > difference (well, at least for now) to do "dd bs=128k" or "dd bs=2MB":
> > the 128kb readahead size will be used underneath to submit read IO.
> > 
> 
> Hmm...
> 
> # echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=128k count=32768
> 32768+0 enregistrements lus
> 32768+0 enregistrements écrits
> 4294967296 octets (4,3 GB) copiés, 20,7718 s, 207 MB/s
> 
> 
> # echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=2M count=2048
> 2048+0 enregistrements lus
> 2048+0 enregistrements écrits
> 4294967296 octets (4,3 GB) copiés, 27,7824 s, 155 MB/s

Interesting. Here are my test results:

root@lkp-nex04 /home/wfg# echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=128k count=32768
32768+0 records in
32768+0 records out
4294967296 bytes (4.3 GB) copied, 19.0121 s, 226 MB/s
root@lkp-nex04 /home/wfg# echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=2M count=2048
2048+0 records in
2048+0 records out
4294967296 bytes (4.3 GB) copied, 19.0214 s, 226 MB/s

Maybe the /dev/sda performance bug on your machine is sensitive to timing?

Thanks,
Fengguang