linux-ext4 - Re: [RFC] Parallelize IO for e2fsck

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Message-ID: <20080118011542.GQ155259@sgi.com>
Date:	Fri, 18 Jan 2008 12:15:42 +1100
From:	David Chinner <dgc@....com>
To:	Valerie Henson <val@...consulting.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger@...sterfs.com>,
	Ric Wheeler <ric@....com>
Subject: Re: [RFC] Parallelize IO for e2fsck

On Wed, Jan 16, 2008 at 01:30:43PM -0800, Valerie Henson wrote:
> Hi y'all,
> 
> This is a request for comments on the rewrite of the e2fsck IO
> parallelization patches I sent out a few months ago.  The mechanism is
> totally different.  Previously IO was parallelized by issuing IOs from
> multiple threads; now a single thread issues fadvise(WILLNEED) and
> then uses read() to complete the IO.

Interesting.

We ultimately rejected a similar patch to xfs_repair (pre-population
the kernel block device cache) mainly because of low memory
performance issues and it doesn't really enable you to do anything
particularly smart with optimising I/O patterns for larger, high
performance RAID arrays.

The low memory problems were particularly bad; the readahead
thrashing cause a slowdown of 2-3x compared to the baseline and
often it was due to the repair process requiring all of memory
to cache stuff it would need later. IIRC, multi-terabyte ext3
filesystems have similar memory usage problems to XFS, so there's
a good chance that this patch will see the same sorts of issues.

> Single disk performance doesn't change, but elapsed time drops by
> about 50% on a big RAID-5 box.  Passes 1 and 2 are parallelized.  Pass
> 5 is left as an exercise for the reader.

Promising results, though....

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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