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    SubjectRe: [PATCH 2/9]: Reduce Log I/O latency
    On Wed, Nov 21, 2007 at 08:57:27PM -0600, Matt Mackall wrote:
    > On Thu, Nov 22, 2007 at 12:12:14PM +1100, David Chinner wrote:
    > > 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.

    Sounds like the PVR is badly designed to me. If a write can cause a
    read to miss a playback deadline, then you haven't built enough
    buffering into your playback application.

    Cheers,

    Dave.
    --
    Dave Chinner
    Principal Engineer
    SGI Australian Software Group
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