Re: [patch] RFC directio: partial writes support

[Nick Piggin <npiggin@suse.de>]

On Tue, Mar 02, 2010 at 12:34:06PM +0100, Jan Kara wrote:
> 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.

OK yes I see, I missed that.