linux-kernel - Re: Higher than expected disk write(2) latency

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Message-Id: <20080709222701.8eab4924.akpm@linux-foundation.org>
Date:	Wed, 9 Jul 2008 22:27:01 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Martin Lucina <mato@...elna.sk>
Cc:	linux-kernel@...r.kernel.org, Martin Sustrik <sustrik@...tmq.com>
Subject: Re: Higher than expected disk write(2) latency

On Sat, 28 Jun 2008 14:11:32 +0200 Martin Lucina <mato@...elna.sk> wrote:

> we're getting some rather high figures for write(2) latency when testing
> synchronous writing to disk.  The test I'm running writes 2000 blocks of
> contiguous data to a raw device, using O_DIRECT and various block sizes
> down to a minimum of 512 bytes.  
> 
> The disk is a Seagate ST380817AS SATA connected to an Intel ICH7
> using ata_piix.  Write caching has been explicitly disabled on the
> drive, and there is no other activity that should affect the test
> results (all system filesystems are on a separate drive).  The system is
> running Debian etch, with a 2.6.24 kernel.
> 
> Observed results:
> 
> size=1024, N=2000, took=4.450788 s, thput=3 mb/s seekc=1
> write: avg=8.388851 max=24.998846 min=8.335624 ms
> 8 ms: 1992 cases
> 9 ms: 2 cases
> 10 ms: 1 cases
> 14 ms: 1 cases
> 16 ms: 3 cases
> 24 ms: 1 cases

stoopid question 1: are you writing to a regular file, or to /dev/sda?  If
the former then metadata fetches will introduce glitches.

stoopid question 2: does the same effect happen with reads?
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