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    SubjectRe: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter

    cache=writeback can be faster than cache=none for the same reasons
    a disk cache speeds up access. As long as the I/O mix contains more
    asynchronous then synchronous writes it allows the host to do much
    more reordering, only limited by the cache size (which can be quite
    huge when using the host pagecache) and the amount of cache flushes
    coming from the host. If you have a fsync heavy workload or metadata
    operation with a filesystem like the current XFS you will get lots
    of cache flushes that make the use of the additional cache limits.

    If you don't have a of lot of cache flushes, e.g. due to dumb
    applications that do not issue fsync, or even run ext3 in it's default
    mode never issues cache flushes the benefit will be enormous, but the
    data loss and possible corruption will be enormous.

    But even for something like btrfs that does provide data integrity
    but issues cache flushes fairly effeciently data=writeback may
    provide a quite nice speedup, especially if using multiple guest
    accessing the same spindle(s).

    But I wouldn't be surprised if IBM's exteme differences are indeed due
    to the extremly unsafe ext3 default behaviour.

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      Last update: 2010-03-16 11:29    [W:0.030 / U:3.340 seconds]
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