linux-ext4 - Re: Enable asynchronous commits by default patch revoked?

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Message-ID: <20090902150306.GF379@mit.edu>
Date:	Wed, 2 Sep 2009 11:03:06 -0400
From:	Theodore Tso <tytso@....edu>
To:	Tom Vier <nester@...il.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: Enable asynchronous commits by default patch revoked?

On Wed, Sep 02, 2009 at 10:48:33AM -0400, Tom Vier wrote:
> On Mon, Aug 24, 2009 at 06:07:39PM -0400, Theodore Tso wrote:
> > Sure, agreed.  But let's look a bit closer at what "async commit"
> > really means.
> > 
> > What ext3 and ext4 does by default is this:
> > 
> > 1)  Write data blocks required by data=ordered mode (if any)
> 
> Shouldn't there be a write barrier after data blocks, so that the journal
> blocks aren't written first? ie, mark the first journal block write with
> barrier flag on.

No, it doesn't matter, because the journal blocks are ignored until
the commit block is written.  So the order of the data blocks required
by data=ordered mode, and the journal blocks are written can be freely
reordered by the elevator and the hard drive without any risk of data
integrity problems.

						- Ted
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