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    SubjectRe: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter
    On Wed, Mar 17, 2010 at 06:22:29PM +0200, Avi Kivity wrote:
    > They should be reorderable. Otherwise host filesystems on several
    > volumes would suffer the same problems.

    They are reordable, just not as extremly as the the page cache.
    Remember that the request queue really is just a relatively small queue
    of outstanding I/O, and that is absolutely intentional. Large scale
    _caching_ is done by the VM in the pagecache, with all the usual aging,
    pressure, etc algorithms applied to it. The block devices have a
    relatively small fixed size request queue associated with it to
    facilitate request merging and limited reordering and having fully
    set up I/O requests for the device.



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