Re: [patch] RFC directio: partial writes support

[Jan Kara <jack@suse.cz>]

On Tue 02-03-10 20:25:02, Nick Piggin wrote:
> On Mon, Mar 01, 2010 at 03:21:49PM -0800, Andrew Morton wrote:
> > On Thu, 25 Feb 2010 15:45:58 +0300
> > Dmitry Monakhov <dmonakhov@openvz.org> wrote:
> > 
> > > Can someone please describe me why directio deny partial writes.
> > > For example if someone try to write 100Mb but file system has less
> > > data it return ENOSPC in the middle of block allocation.
> > > All allocated blocks will be truncated (it may be 100Mb -4k) end
> > > ENOSPC will be returned. As far as i remember direct_io always act
> > > like this, but i never asked why?
> > > Why do we have to give up all the progress we made?
> > > In fact partial writes are possible in case of holes, when we 
> > > fall back to buffered write. XFS implemented partial writes.
> > 
> > The problem with direct-io writes is that the writes don't necessarily
> > complete in file-offset-ascending order.  So if we've issued 50 write
> > BIOs and then hit an EIO on a BIO then we could have a hunk of
> > unwritten data with newly-writted data either side of it.  If we get a
> > bunch of discontiguous EIO BIOs coming in then the problem gets even
> > messier - we have a span of disk which has a random mix of
> > correctly-written and not-correctly-written runs of sectors.  What do
> > we do with that?
> 
> Hmm, what if we're filling in a hole with direct IO? I don't see where
> blocks allocated in DIO code will be trimmed on a failed write (because
> it's within isize). This could cause uninitalized data of the block to
> leak couldn't it?
  The trick is that blockdev_direct_IO is defined to pass
DIO_SKIP_HOLES to __blockdev_direct_IO. Thus e.g. ext2 or ext3 will just
fail the direct IO if there is a hole and we fall back to buffered IO
which should handle that just fine.

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