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

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Message-ID: <20090825001554.GN17684@mit.edu>
Date:	Mon, 24 Aug 2009 20:15:54 -0400
From:	Theodore Tso <tytso@....edu>
To:	Andreas Dilger <adilger@....com>
Cc:	Ric Wheeler <rwheeler@...hat.com>,
	Christian Fischer <Christian.Fischer@...terngraphics.com>,
	linux-ext4@...r.kernel.org
Subject: Re: Enable asynchronous commits by default patch revoked?

On Mon, Aug 24, 2009 at 05:43:36PM -0600, Andreas Dilger wrote:
> Without transaction checksums waiting on all of the blocks together
> is NOT safe.  If the commit record is on disk, but the rest of the
> transaction's blocks are not then during replay it may cause garbage
> to be written from the journal into the filesystem metadata.

Yes, I *said* that we can only wait on all of the blocks together with
the commit record when doing journal checksums.  Sorry if I didn't
make that clear enough.

That's the one optimization we using journal checksums buys us.
Unfortunately it does not allow us to omit the barrier
operation.... and have real-world testing experience that without the
barrier, a power drop can cause significant filesystem corruption and
potential data loss.

Try using Chris Mason's torture-test workload with async-checksums
without this patch; you will get data corruption if you try dropping
power while his torture-test is running.  I know you really don't like
the barrier, but I'm afraid it's not safe to run without it, even with
journal checksums.

						- Ted
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