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    SubjectRe: [patch/rft] jbd2: tag journal writes as metadata I/O
    On Thu, Apr 01 2010, Jeff Moyer wrote:
    > Hi,
    >
    > In running iozone for writes to small files, we noticed a pretty big
    > discrepency between the performance of the deadline and cfq I/O
    > schedulers. Investigation showed that I/O was being issued from 2
    > different contexts: the iozone process itself, and the jbd2/sdh-8 thread
    > (as expected). Because of the way cfq performs slice idling, the delays
    > introduced between the metadata and data I/Os were significant. For
    > example, cfq would see about 7MB/s versus deadline's 35 for the same
    > workload. I also tested fs_mark with writing and fsyncing 1000 64k
    > files, and a similar 5x performance difference was observed. Eric
    > Sandeen suggested that I flag the journal writes as metadata, and once I
    > did that, the performance difference went away completely (cfq has
    > special logic to prioritize metadata I/O).
    >
    > So, I'm submitting this patch for comments and testing. I have a
    > similar patch for jbd that I will submit if folks agree that this is a
    > good idea.

    Looks good to me.

    --
    Jens Axboe



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