Re: [PATCH 4/4] ext4: call blkdev_issue_flush on fsync
From: Jamie Lokier
Date: Tue May 20 2008 - 11:44:10 EST
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.
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