Re: Correct behavior on O_DIRECT sparse file writes

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 12 Oct 2007 16:39:27 -0400
Chris Mason <chris.mason@oracle.com> wrote:

> Hello everyone,
> 
> The test below creates a sparse file and then fills a hole with
> O_DIRECT.  As far as I can tell from reading generic_osync_inode, the
> filesystem metadata is only forced to disk if i_size changes during the
> file write.  I've tested ext3, xfs and reiserfs and they all skip the
> commit when filling holes.
> 
> I would argue that filling holes via O_DIRECT is supposed to commit the
> metadata required to find those file blocks later.  At least on ext3,
> O_SYNC does force a commit on fill holes  (haven't tested others).
> 
> So, is the current behavior a bug or a feature?

I don't think it's a bug.  Sure, O_DIRECT is synchronous, but that's
because it is, err, direct.  Not because it provides extra data-integrity
guarantees.  If you want those guarantees, use O_SYNC as well.

> dd if=/dev/zero of=foo bs=1M seek=1 count=1 oflag=direct
> 
> hexdump foo | head -n 2
> 0000000 62b1 ea2d 73e8 c64f f5ef 1af5 dd09 8ccd
> 0000010 75ec 9581 e0ea ae9b e28f b76d a700 4d5b
> 
> dd if=/dev/urandom of=foo bs=4k count=1 conv=notrunc oflag=direct
> reboot -nf
> 
> (after reboot)
> 
> hexdump foo
> 0000000 0000 0000 0000 0000 0000 0000 0000 0000
> *
> 0200000
> 
> -chris
>