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    SubjectRe: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority
    On Mon, Oct 06 2008, Dave Chinner wrote:
    > On Sat, Oct 04, 2008 at 05:45:00PM +1000, Aaron Carroll wrote:
    > > Dave Chinner wrote:
    > >> On Thu, Oct 02, 2008 at 05:32:04PM +0200, Bodo Eggert wrote:
    > >>> Sounds like you need a priority class besides sync and async.
    > >>
    > >> There's BIO_META now as well, which I was testing at the same time
    > >> as RT priority. Marking all the metadata I/O as BIO_META did help,
    > >> but once again I never got to determining if that was a result of
    > >> the different tagging or the priority increase.
    > >
    > > What exactly do you want META to mean? Strict prioritisation over
    > > all other non-META requests, or just more frequent and/or larger
    > > dispatches? Should META requests be sorted?
    > The real question is "what was it supposed to mean"? AFAICT, it was
    > added to a couple of filesystems to be used to tag superblock read
    > I/O. Why - I don't know - there's a distinct lack of documentation
    > surrounding these bio flags. :/

    It was added to be able to differentiate between data and meta data IO
    when using blktrace, that is all.

    > Realistically, I'm not sure that having a separate queue for
    > BIO_META will buy us anything, given that noop is quite often the
    > fastest scheduler for XFS because it enables interleaved metadata
    > I/O to be merged with data I/O. Like I said, I was not able to spend
    > the time to determine exactly how BIO_META affected I/O patterns, so
    > I can't really comment on whether it is really necessary or not.

    There's no seperate queue for meta data IO anywhere. CFQ will give
    _slight_ preference to meta data IO as a side effect, preferring the
    meta IO for otherwise same IO in what to serve next in the same queue.
    And it will not allow preemption of a meta data IO for a data IO.

    So using meta should not yield any important boosts by itself.

    Jens Axboe

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