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    SubjectRe: [PATCH 4/4] block: Optionally snapshot page contents to provide stable pages during write
    On Thu, Dec 13, 2012 at 06:10:49PM -0800, Darrick J. Wong wrote:
    > On Thu, Dec 13, 2012 at 05:48:06PM -0800, Andy Lutomirski wrote:
    > > On 12/13/2012 12:08 AM, Darrick J. Wong wrote:
    > > > Several complaints have been received regarding long file write latencies when
    > > > memory pages must be held stable during writeback. Since it might not be
    > > > acceptable to stall programs for the entire duration of a page write (which may
    > > > take many milliseconds even on good hardware), enable a second strategy wherein
    > > > pages are snapshotted as part of submit_bio; the snapshot can be held stable
    > > > while writes continue.
    > > >
    > > > This provides a band-aid to provide stable page writes on jbd without needing
    > > > to backport the fixed locking scheme in jbd2. A mount option is added to ext4
    > > > to allow administrators to enable it there.
    > >
    > > I'm a bit confused as to what it has to do with ext3. Wouldn't this be
    > > useful as a mount option everywhere, though?
    >
    > ext3 requires snapshots; the rest are ok with either strategy.
    >
    > *If* snapshotting is generally liked, then yes I'll go redo it as a vfs mount
    > option.

    It's copying every single IO, right? If so, then please don't
    propagate any further than is necessary to fix the broken
    filesystems...

    Cheers,

    Dave.
    --
    Dave Chinner
    david@fromorbit.com


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