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    SubjectRe: [PATCH] PAGE_SIZE IO for RAW (RAW VARY)
    On Thu, Jan 10, 2002 at 11:51:51AM +0100, Jens Axboe wrote:
    > On Thu, Jan 10 2002, Andrea Arcangeli wrote:
    > > On Thu, Jan 10, 2002 at 11:22:25AM +0100, Jens Axboe wrote:
    > > > On Thu, Jan 10 2002, Andrea Arcangeli wrote:
    > > > > On Wed, Jan 09, 2002 at 11:28:39AM -0800, Badari Pulavarty wrote:
    > > > > > Ben,
    > > > > >
    > > > > > By any chance do you have a list of drivers that assume this ?
    > > > > > What does it take to fix them ?
    > > > > >
    > > > > > I think Jens BIO changes for 2.5 will fix this problem. But 2.4
    > > > > > needs a solution in this area too. This patch showed significant
    > > > > > improvement for database workloads.
    > > > >
    > > > > I didn't checked the implementation but as far as the blkdev is
    > > > > concerned the b_size changes without notification as soon as you
    > > > > 'mkfs -b somethingelse' and then mount the fs. So it cannot break
    > > > > as far I can tell. The only important thing is that b_size stays
    > > > > between 512 and 4k.
    > > >
    > > > The concern is/was differently sized buffer_heads in the same
    > > > request, ie b_size changing as you iterate through the chunks of one
    > > > request.
    > >
    > > ok, I don't expect problems there. It can happen for example if you
    > > create a snapshot with 4k and then you switch back the original volume
    > > to 1k. the physical volume will get mixed b_size colaesced into the
    > > same request.
    >
    > Well I don't expect problems either, however Jeff Merkey did report them
    > but see my previous mail on that (validity of that report is
    > questionable).
    >
    > I still wouldn't feel to good doing this, and just because snapshotting
    > opens the possibility for this to happen doesn't mean it a) ever
    > triggered in real life, and b) works on all devices.

    fair enough. one way to do it certainly safely is to add a bitflag to
    the struct blkkdev.

    Andrea
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