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    SubjectRe: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback_pages
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    On Tue, 2009-09-01 at 13:19 +0200, Jens Axboe wrote:

    > Originally, MAX_WRITEBACK_PAGES was hard-coded to 1024 because of a
    > concern of not holding I_SYNC for too long. (At least, that was the
    > comment previously.) This doesn't make sense now because the only
    > time we wait for I_SYNC is if we are calling sync or fsync, and in
    > that case we need to write out all of the data anyway. Previously
    > there may have been other code paths that waited on I_SYNC, but not
    > any more.
    >
    > According to Christoph, the current writeback size is way too small,
    > and XFS had a hack that bumped out nr_to_write to four times the value
    > sent by the VM to be able to saturate medium-sized RAID arrays. This
    > value was also problematic for ext4 as well, as it caused large files
    > to be come interleaved on disk by in 8 megabyte chunks (we bumped up
    > the nr_to_write by a factor of two).
    >
    > So, in this patch, we make the MAX_WRITEBACK_PAGES a tunable, and
    > change the default to be 32768 blocks.

    Do we really need a tunable for this?

    I guess we need a limit to avoid it writing out everything, but can't we
    have something automagic?



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