linux-kernel - Re: [PATCH 4/4] ext4: call blkdev_issue

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Message-ID: <20080520154313.GI16676@shareable.org>
Date:	Tue, 20 May 2008 16:43:14 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Theodore Tso <tytso@....edu>, Eric Sandeen <sandeen@...hat.com>,
	ext4 development <linux-ext4@...r.kernel.org>,
	linux-kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 4/4] ext4: call blkdev_issue_flush on fsync

Theodore Tso wrote:
> On Fri, May 16, 2008 at 02:09:56PM -0500, Eric Sandeen wrote:
> > To ensure that bits are truly on-disk after an fsync,
> > we should call blkdev_issue_flush if barriers are supported.
> 
> This patch isn't necessary, and in fact will cause a double flush.
> When you call fsync(), it calls ext4_force_commit(), and we do a the
> equivalent of a blkdev_issue_flush() today (which is what happenes
> when you do a submit_bh(WRITE_BARRIER, bh), which is what setting
> set_ordered_mode(bh) ends up causing.

ISTR fsync() on ext3 did not always force a commit, if in-place data
writes did not change any metadata.  Has this been fixed in ext4 but
not ext3 then?

Does WRITE_BARRIER always cause a flush?  It does not have to
according to Documentation/block/barrier.txt.  There are caveats about
tagged queuing "not yet implemented" in the text, but can we rely on
that?  The documentation is older than the current implementation;
those caveats might no longer apply.

-- Jamie
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