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    SubjectRe: Linux 2.6.29
    Ric Wheeler wrote:
    > Linus Torvalds wrote:
    >> That's one of the issues. The cost of those flushes can be really
    >> quite high, and as mentioned, in the absense of redundancy you don't
    >> actually get the guarantees that you seem to think that you get.
    > I have measured the costs of the write flushes on a variety of devices,
    > routinely, a cache flush is on the order of 10-20 ms with a healthy
    > s-ata drive.

    Err, no. Yes, the flush itself will be very quick,
    since the drive is nearly always keeping up with the I/O
    already (as we are discussing in a separate subthread here!).

    But.. the cost of that FLUSH_CACHE command can be quite significant.
    To issue it, we first have to stop accepting R/W requests,
    and then wait for up to 32 of them currently in-flight to complete.
    Then issue the cache-flush, and wait for that to complete.

    Then resume R/W again.

    And FLUSH_CACHE is a PIO command for most libata hosts,
    so it has a multi-microsecond CPU hit as well as the I/O hit,
    whereas regular R/W commands will usually use less CPU because
    they are usually done via an automated host command queue.

    Tiny, but significant. And more so on smaller/slower end-user systems
    like netbooks than on datacenter servers, perhaps.


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