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

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Date:	Mon, 24 Aug 2009 12:31:19 -0600
From:	Andreas Dilger <adilger@....com>
To:	Theodore Tso <tytso@....edu>
Cc:	Christian Fischer <Christian.Fischer@...terngraphics.com>,
	linux-ext4@...r.kernel.org
Subject: Re: Enable asynchronous commits by default patch revoked?

On Aug 24, 2009  09:34 -0400, Theodore Ts'o wrote:
> On Mon, Aug 24, 2009 at 10:33:10AM +0200, Christian Fischer wrote:
> > I try to figure out reasonable mount options for ext4.
> > 
> > I've seen a "Enable asynchronous commits by default" patch from Sun, 21 Sep 
> > 2008.
> > 
> > Why is it revoked?
> 
> It patch was never merged because the ayschronous commits feature
> disabled all write barriers, so under heavy workloads a power failure
> could cause data loss.
> 
> No one has gotten around to looking at this closely; I think adding a
> strategically placed blkdev_issue_flush() will allow us to safely
> enable this feature, but it needs careful study.

I don't think that was the issue, but rather that we wanted to have
per-block checksums in order to handle the case were some block in
transaction A is causing a transaction checksum failure, yet transaction
B has already committed and begun checkpointing.

One option discussed was to add a lightweight 16-bit checksum (e.g. TCP
checksum) to the high bits of the t_flags of the block tag.  The checksum
doesn't have to be very strong since the whole-transaction checksum will
be the primary point of validation.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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