linux-kernel - Re: Bad SSD performance with recent kernels

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Date:	Sun, 29 Jan 2012 14:13:51 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Wu Fengguang <wfg@...ux.intel.com>
Cc:	Herbert Poetzl <herbert@...hfloor.at>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>
Subject: Re: Bad SSD performance with recent kernels

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



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