linux-ext4 - Re: async commit & write barrier code

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Date:	Tue, 23 Sep 2008 15:41:02 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Ric Wheeler <rwheeler@...hat.com>
CC:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: async commit & write barrier code

Ric Wheeler wrote:
> After today's call, I was poking around a bit to try and understand 
> better how the async commit code plays with the write barrier.
> 
> journal_submit_commit_record seems to disable the barriers when async IO 
> is enabled if I read the code correctly. If this is true, how can we 
> provide any promises of on disk data integrity after an fsync()?
> 
> Regards,
> 
> Ric

I agree; with async commit, ext4/jbd2 is running with *no* barrier
writes in jbd code. (FWIW, on the fsync front, fsync calls
blkdev_issue_flush in ext4 so that part may actually be ok in the end).

But at a minimum, I think that for data=ordered, there is now *no*
guarantee that the associated file data actually hits disk before the
size updates, is there?

-Eric
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