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    SubjectRe: [PATCH 6/6] writeback: limit write_cache_pages integrity scanning to current EOF
    On Tue, Jun 08, 2010 at 03:38:31PM +1000, Nick Piggin wrote:
    > On Tue, Jun 08, 2010 at 10:38:07AM +1000, Dave Chinner wrote:
    > > From: Dave Chinner <dchinner@redhat.com>
    > >
    > > sync can currently take a really long time if a concurrent writer is
    > > extending a file. The problem is that the dirty pages on the address
    > > space grow in the same direction as write_cache_pages scans, so if
    > > the writer keeps ahead of writeback, the writeback will not
    > > terminate until the writer stops adding dirty pages.
    > >
    > > For a data integrity sync, we only need to write the pages dirty at
    > > the time we start the writeback, so we can stop scanning once we get
    > > to the page that was at the end of the file at the time the scan
    > > started.
    > >
    > > This will prevent operations like copying a large file preventing
    > > sync from completing as it will not write back pages that were
    > > dirtied after the sync was started. This does not impact the
    > > existing integrity guarantees, as any dirty page (old or new)
    > > within the EOF range at the start of the scan will still be
    > > captured.
    > >
    > > This patch will not prevent sync from blocking on large writes into
    > > holes.
    >
    > The writes don't have to be into holes to cause this starvation
    > problem, do they?

    No, they don't.

    > > That requires more complex intervention while this patch only
    > > addresses the common append-case of this sync holdoff.
    >
    > Jan's tagging patch looks pretty good to me and isn't so complex.
    > I think we should just take that.

    I don't care which one we take as long as it is actually tested by
    more than the submitter and we get everything in for 2.6.35...

    Cheers,

    Dave.
    --
    Dave Chinner
    david@fromorbit.com


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