[Cluster-devel] fallocate vs O_(D)SYNC

[Jan Kara]

  Hello,

On Wed 16-11-11 09:43:08, Steven Whitehouse wrote:
> On Wed, 2011-11-16 at 03:42 -0500, Christoph Hellwig wrote:
> > It seems all filesystems but XFS ignore O_SYNC for fallocate, and never
> > make sure the size update transaction made it to disk.
> > 
> > Given that a fallocate without FALLOC_FL_KEEP_SIZE very much is a data
> > operation (it adds new blocks that return zeroes) that seems like a
> > fairly nasty surprise for O_SYNC users.
> 
> In GFS2 we zero out the data blocks as we go (since our metadata doesn't
> allow us to mark blocks as zeroed at alloc time) and also because we are
> mostly interested in being able to do FALLOC_FL_KEEP_SIZE which we use
> on our rindex system file in order to ensure that there is always enough
> space to expand a filesystem.
> 
> So there is no danger of having non-zeroed blocks appearing later, as
> that is done before the metadata change.
> 
> Our fallocate_chunk() function calls mark_inode_dirty(inode) on each
> call, so that fsync should pick that up and ensure that the metadata has
> been written back. So we should thus have both data and metadata stable
> on disk.
> 
> Do you have some evidence that this is not happening?
  Yeah, only that nobody calls that fsync() automatically if the fd is
O_SYNC if I'm right. But maybe calling fdatasync() on the range which was
fallocated from sys_fallocate() if the fd is O_SYNC would do the trick for
most filesystems? That would match how we treat O_SYNC for other operations
as well. I'm just not sure whether XFS wouldn't take unnecessarily big hit
with this.

								Honza
-- 
Jan Kara <jack at suse.cz>
SUSE Labs, CR