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    SubjectRe: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority
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    Jens Axboe <jens.axboe@oracle.com> wrote:
    > On Thu, Oct 02 2008, Dave Chinner wrote:
    >> On Thu, Oct 02, 2008 at 09:55:11AM +0200, Jens Axboe wrote:

    >> > Good point. I think we should mark the IO as sync, and maintain the same
    >> > priority level. Any IO that ends up being waited on is sync by
    >> > definition, we just need to expand the coverage a bit.
    >>
    >> That's what XFS has always done - mark the journal I/O as sync.
    >> Still, once you load up the elevator, the sync I/O can still get
    >> delayed for hundreds of milliseconds before dispatch, which was
    >> why I started looking at boosting the priority of the log I/O.
    >> It proved to be much more effective at getting the log I/O
    >> dispatched than the existing "mark it sync" technique....
    >
    > Sure, just marking it as sync is not a magic bullet. It'll be in the
    > first priority for that class, but it'll share bandwidth with other
    > processes. So if you have lots of IO going on, it can take hundreds of
    > miliseconds before being dispatched.

    Sounds like you need a priority class besides sync and async.




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