linux-kernel - Re: Flash IO slow 1.5 MB/s

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Message-ID: <u2m58965d8a1005041034h72605714z46a1a1e06eccc86c@mail.gmail.com>
Date:	Tue, 4 May 2010 12:34:52 -0500
From:	Paul Hartman <paul.hartman+linux@...il.com>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Flash IO slow 1.5 MB/s

On Mon, May 3, 2010 at 10:52 PM, Trenton D. Adams
<trenton.d.adams@...il.com> wrote:
> It really looks like there's a scheduling issue.  It seems as if the
> system is IO thrashing on the flash drive, and bounces all over the
> place in terms of performance.  Sometimes it's really low, like the
> 2.73M/s, and other times it's really fast, like the 28.86M/s.
> Although you can't see it there, there were times when rsync was
> registering 200kb/s.  None of them are "really" accurate, as
> everything is queued for writing, but the final results of 1.5M/s
> (calculated from the "real" time) is terrible.

I have a similar experience (posted to this list a few months ago)
with mounting a flash device (mobile phone) in USB mass storage mode.
When I/O scheduler for that device is CFQ, write performance is really
terrible. When I change the scheduler to deadline, performance is
several times better. In 2.6.32 pdflush was replaced and CFQ
performance saw a 4x increase but still far too slow.

CFQ in <=2.6.31: 450KB/sec
CFQ in >=2.6.32: 2MB/sec
Deadline in all: 17MB/sec

I didn't try anything with dirty_bytes.

FWIW :)
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