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    SubjectRe: real-time threaded IO with low latency (audio)
    Jason Gunthorpe wrote:
    > Some time ago on the QNX news groups someone was having this problem with
    > QNX - they wanted to stream video onto disk at very nearly the full
    > bandwidth of the drive (8Meg/sec I think). They found that QNX faultered
    > and designed a test case that measured the best/worst/average for a write
    > call - turned out the worst case was just awefull - 2-3 times larger then
    > their upper limit and much larger than the average case.
    > I don't know exactly what the QNX developers did, but they actually did
    > fix it, they brought the worst case for write down into an acceptable
    > region. It was about at this time that their pre-allocation scheme showed
    > up in the file system so maybe that had something to do with it.

    Pre-allocation sound like a very probably reason to me, it least if the
    problem was related to streaming multiple files simultaneously, or even
    streaming to a disk containing other files. How to avoid fragmentation
    otherwise? (In the case of this kind of applications, a small fragment
    is a few hundred kB...)

    > They must have some resticitions, a largely unfragemented drive, AV disks
    > and only a single application are some likely candiates. But the people
    > who needed this simply accepted those limitations - I imagine if you
    > wanted to do this on linux you'd start with a dedicated AV drive (no ecc,
    > or thermal recalibrations - ment for realtime video streaming) that didn't
    > have swap or the root filesystem on it..
    > Jason


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