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    SubjectRe: Lower HD transfer rate with NCQ enabled?
    Phillip Susi wrote:
    > Mark Lord wrote:
    >> Phillip Susi wrote:
    >>> Sounds like this is a serious bug in the WD firmware.
    >> For personal systems, yes. For servers, probably not a bug.
    >> Disabling readahead means faster execution queued commands,
    >> since it doesn't have to "linger" and do unwanted read-ahead.
    >> So this bug is a "feature" for random access servers.
    >> And a big nuisance for everything else.
    > I think you misunderstand the bug. The bug is not that the drive
    > disables internal readahead; the bug is that host supplied readahead
    > requests work so horribly. It is a good thing that the drive allows the
    > host to control the readahead, but something is wrong if the drive's
    > readahead is WAY better than any the host can perform.

    Well, in this case, it has already been determined that switching
    to a different Linux I/O scheduler gives back most of the performance.

    But the drive can do readahead better than the OS: With the OS,
    everything is broken up into discrete requests, whereas with the
    drive firmware, it can continuously update it's readahead projections,
    even in the midst of a command. So it does have an advantage.

    But again, only the WD Raptor seems to have serious problems here.
    Other drives cope well with readahead + NCQ just fine.

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      Last update: 2007-04-10 06:01    [W:0.022 / U:9.244 seconds]
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