linux-ext4 - Re: [PATCH 0/4] (RESEND) ext3[34] barrier changes

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Message-ID: <20080520234417.GL27853@shareable.org>
Date:	Wed, 21 May 2008 00:44:17 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>,
	Eric Sandeen <sandeen@...hat.com>, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/4] (RESEND) ext3[34] barrier changes

Chris Mason wrote:
> write log blocks
> flush #1
> write commit block
> flush #2
> write metadata blocks
> 
> I'd agree with Ted, there's a fairly small chance of things get reordered 
> around flush #1.

Except when it crosses an MD disk boundary.  Then it's really likely.

We could also ask if there's _any_ possibility, when they are a merged
single I/O, of them not getting written in the expected order?
What about when FUA is set, does that imply any order?

But it's all moot: Checksumming is the way forward here, no doubt.
Checksumming makes the multi-sector write "atomic or corrupt".  That's
the same expectation as a commit sector provides by itself, but
generalised to the whole journal entry.

-- Jamie
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