Re: [patch] RFC directio: partial writes support

[Nick Piggin <npiggin@suse.de>]

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?