linux-kernel - Re: ext2 + -osync: not as easy as it seems

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Date:	Wed, 14 Jan 2009 09:12:04 -0500
From:	Theodore Tso <tytso@....EDU>
To:	Jan Kara <jack@...e.cz>
Cc:	Fernando Luis Vázquez Cao 
	<fernando@....ntt.co.jp>, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Pavel Machek <pavel@...e.cz>,
	kernel list <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>, sandeen@...hat.com
Subject: Re: ext2 + -osync: not as easy as it seems

On Wed, Jan 14, 2009 at 03:05:32PM +0100, Jan Kara wrote:
> On Wed 14-01-09 08:21:46, Theodore Tso wrote:
> > 
> > If we optimize out the journal commit when there are no blocks
> > attached to the transaction, we could change the patch to only force a
> > flush if inode->i_state did not have I_DIRTY before the call to
> > sync_inode().  Does that sound sane?
>   Yes. And also add a flush in case of fdatasync().

Um, we have that already; the sync_inode() followed by
blkdev_issue_flush() is the path taken by fdatasync(), I do believe.

>   Well, I thought that a barrier, as an abstraction, only guarantees that
> any IO which happened before the barrier hits the iron before any IO which
> has been submitted after a barrier. This is actually enough for a
> journalling to work correctly but it's not enough for fsync() guarantees.
> But I might be wrong...

Ah, yes, that's what you're getting at.  True, but for better or for
worse, we have no other interface other than blkdev_issue_flush().
This will guarantee that the data has made it to the disk controller,
but it won't necessarily guarantee that it will have made it onto the
disk platter, as I understand things; but I don't think we have any
other interfaces available to us at this point.

						- Ted
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