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    SubjectRe: [PATCH 2/9]: Reduce Log I/O latency
    On Thu, Nov 22, 2007 at 12:12:14PM +1100, David Chinner wrote:
    > On Thu, Nov 22, 2007 at 01:49:25AM +0100, Andi Kleen wrote:
    > > David Chinner <dgc@sgi.com> writes:
    > >
    > > > To ensure that log I/O is issued as the highest priority I/O, set
    > > > the I/O priority of the log I/O to the highest possible. This will
    > > > ensure that log I/O is not held up behind bulk data or other
    > > > metadata I/O as delaying log I/O can pause the entire transaction
    > > > subsystem. Introduce a new buffer flag to allow us to tag the log
    > > > buffers so we can discrimiate when issuing the I/O.
    > >
    > > Won't that possible disturb other RT priority users that do not need
    > > log IO (e.g. working on preallocated files)? Seems a little
    > > dangerous.
    >
    > In all the cases that I know of where ppl are using what could
    > be considered real-time I/O (e.g. media environments where they
    > do real-time ingest and playout from the same filesystem) the
    > real-time ingest processes create the files and do pre-allocation
    > before doing their I/O. This I/O can get held up behind another
    > process that is not real time that has issued log I/O.
    >
    > Given there is no I/O priority inheritence and having log I/O stall
    > will stall the entire filesystem, we cannot allow log I/O to
    > stall in real-time environments. Hence it must have the highest
    > possible priority to prevent this.

    I've seen PVRs that would be upset by this. They put media on one
    filesystem and database/apps/swap/etc. on another, but have everything
    on a single spindle. Stalling a media filesystem read for a write
    anywhere else = fail.

    --
    Mathematics is the supreme nostalgia of our time.
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