linux-ext4 - Re: [PATCH 0/2] ext3 HACKs

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Message-ID: <20090714155118.GB10131@mit.edu>
Date:	Tue, 14 Jul 2009 11:51:18 -0400
From:	Theodore Tso <tytso@....edu>
To:	Adrian Hunter <adrian.hunter@...ia.com>
Cc:	Andrew.Morton.akpm@...ux-foundation.org,
	Andreas.Dilger.adilger@....com, Stephen.Tweedie.sct@...hat.com,
	Artem Bityutskiy <artem.bityutskiy@...ia.com>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 0/2] ext3 HACKs

On Tue, Jul 14, 2009 at 05:02:53PM +0300, Adrian Hunter wrote:
> Hi
> 
> We are using linux 2.6.28 and we have a situation where ext3
> can take 30-60 seconds to mount.
> 
> The cause is the underlying device has extremely poor random
> write speed (several orders of magnitude slower than sequential
> write speed), and journal recovery can involve many small random
> writes.
> 
> To alleviate this situation somewhat, I have two moderately ugly
> hacks:
> 	HACK 1: ext3: mount fast even when recovering
> 	HACK 2: do I/O read requests while ext3 journal recovers
> 
> HACK 1 uses a I/O barrier in place of waiting for recovery I/O to be
> flushed.
> 
> HACK 2 crudely throws I/O read requests to the front of the dispatch
> queue until the I/O barrier from HACK 1 is reached.

Have you actually benchmarked these patches, ideally with a fixed
filesystem image so the two runs are done requiring exactly the same
number of blocks to recover?  We implement ordered I/O in terms of
doing a flush, so it would be surprising to see that a significant
difference in times.  Also, it would be useful to do a blktrace before
and after your patches, again with a fixed filesystem image so the
experiment can be carefully controlled.

Regards,

						- Ted
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