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    SubjectRe: [RFC] nfs: use 2*rsize readahead size
    On Wed, Feb 24, 2010 at 07:18:26PM +0800, Akshat Aranya wrote:
    > On Wed, Feb 24, 2010 at 12:22 AM, Dave Chinner <> wrote:
    > >
    > >> It sounds silly to have
    > >>
    > >>         client_readahead_size > server_readahead_size
    > >
    > > I don't think it is  - the client readahead has to take into account
    > > the network latency as well as the server latency. e.g. a network
    > > with a high bandwidth but high latency is going to need much more
    > > client side readahead than a high bandwidth, low latency network to
    > > get the same throughput. Hence it is not uncommon to see larger
    > > readahead windows on network clients than for local disk access.
    > >
    > > Also, the NFS server may not even be able to detect sequential IO
    > > patterns because of the combined access patterns from the clients,
    > > and so the only effective readahead might be what the clients
    > > issue....
    > >
    > In my experiments, I have observed that the server-side readahead
    > shuts off rather quickly even with a single client because the client
    > readahead causes multiple pending read RPCs on the server which are
    > then serviced in random order and the pattern observed by the
    > underlying file system is non-sequential. In our file system, we had
    > to override what the VFS thought was a random workload and continue to
    > do readahead anyway.

    What's the server side kernel version, plus client/server side
    readahead size? I'd expect the context readahead to handle it well.

    With the patchset in <>, you can
    actually see the readahead details:

    # echo 1 > /debug/tracing/events/readahead/enable
    # cp test-file /dev/null
    # cat /debug/tracing/trace # trimmed output
    readahead-initial(dev=0:15, ino=100177, req=0+2, ra=0+4-2, async=0) = 4
    readahead-subsequent(dev=0:15, ino=100177, req=2+2, ra=4+8-8, async=1) = 8
    readahead-subsequent(dev=0:15, ino=100177, req=4+2, ra=12+16-16, async=1) = 16
    readahead-subsequent(dev=0:15, ino=100177, req=12+2, ra=28+32-32, async=1) = 32
    readahead-subsequent(dev=0:15, ino=100177, req=28+2, ra=60+60-60, async=1) = 24
    readahead-subsequent(dev=0:15, ino=100177, req=60+2, ra=120+60-60, async=1) = 0

    And I've actually verified the NFS case with the help of such traces
    long ago. When client_readahead_size <= server_readahead_size, the
    readahead requests may look a bit random at first, and then will
    quickly turn into a perfect series of sequential context readaheads.

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      Last update: 2010-02-25 13:41    [W:0.023 / U:5.684 seconds]
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