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    SubjectRe: [patch] RFC directio: partial writes support
    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?



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