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    SubjectRe: [PATCH][RFC] fast file mapping for loop
    On Thu, Jan 10 2008, Nick Piggin wrote:
    > On Wednesday 09 January 2008 19:52, Jens Axboe wrote:
    >
    > > So how does it work? Instead of punting IO to a thread and passing it
    > > through the page cache, we instead attempt to send the IO directly to the
    > > filesystem block that it maps to.
    >
    > You told Christoph that just using direct-IO from kernel still doesn't
    > give you the required behaviour... What about queueing the IO directly
    > *and* using direct-IO? I guess it still has to go through the underlying
    > filesystem, but that's probably a good thing.

    If it was O_DIRECT and aio, then we would be close.

    > > loop maintains a prio tree of known
    > > extents in the file (populated lazily on demand, as needed).
    >
    > Just a quick question (I haven't looked closely at the code): how come
    > you are using a prio tree for extents? I don't think they could be
    > overlapping?

    Because I'm really lazy - the core of this was basically first written
    as a quick hack and then I always go shopping for reusable data
    structures. prio trees fit the bill nicely, they described extents and
    allowed loopup with a key anywhere in that extent. You are right in that
    I don't need the overlap handling at all, and Chris already tried to
    talk me into reusing his btrfs extent code :-)

    So I may just do the latter, turning it into a lib/extent-map.c in the
    longer run. My first priority was just having something that worked so I
    could test it. At the end of the day, not a single soul would ever
    notice if the prio tree ended up being slightly slower than a custom
    solution.

    --
    Jens Axboe



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