From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: JFYI: ext4 bug triggerable by kvm
Date: Tue, 17 Aug 2010 05:07:55 -0400
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Cc: Christoph Hellwig <hch@infradead.org>,
	Michael Tokarev <mjt@tls.msk.ru>,
	KVM list <kvm@vger.kernel.org>, Kevin Wolf <kwolf@redhat.com>
To: Anthony Liguori <anthony@codemonkey.ws>
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On Mon, Aug 16, 2010 at 03:34:12PM -0500, Anthony Liguori wrote:
> On 08/16/2010 01:42 PM, Christoph Hellwig wrote:
> >On Mon, Aug 16, 2010 at 09:43:09AM -0500, Anthony Liguori wrote:
> >>>Also, ext4 is _very_ slow on O_SYNC writes (which is
> >>>used in kvm with default cache).
> >>Yeah, we probably need to switch to sync_file_range() to avoid the
> >>journal commit on every write.
> >>
> >No, we don't.  sync_file_range does not actually provide any data
> >integrity.
> 
> What do you mean by data integrity?

sync_file_range only does pagecache-level writeout of the file data.
It nevers calls into the actual filesystem, that means any block
allocations (for filling holes / converting preallocated space in normal
filesystems, or every write in COW-based filesstems like qcow2) never
get flushes to disk, and even more importantly the disk write cache is
never flushed.

In short it's completely worthless for any real filesystem.