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    SubjectRe: Why is NCQ enabled by default by libata? (2.6.20)
    Mark Rustad wrote:
    > reorder any queued operations. Of course if you really care about your
    > data, you don't really want to turn write cache on.

    That's a gross exaggeration. FLUSH CACHE and FUA both ensure data
    integrity as well.

    Turning write cache off has always been a performance-killing action on ATA.

    > Also the controller used can have unfortunate interactions. For example
    > the Adaptec SAS controller firmware will never issue more than two
    > queued commands to a SATA drive (even though the firmware will happily
    > accept more from the driver), so even if an attached drive is capable of
    > reordering queued commands, its performance is seriously crippled by not
    > getting more commands queued up. In addition, some drive firmware seems
    > to try to bunch up queued command completions which interacts very badly
    > with a controller that queues up so few commands. In this case turning
    > NCQ off performs better because the drive knows it can't hold off
    > completions to reduce interrupt load on the host – a good idea gone
    > totally wrong when used with the Adaptec controller.

    All of that can be fixed with an Adaptec firmware upgrade, so not our
    problem here, and not a reason to disable NCQ in libata core.

    > Today SATA NCQ seems to be an area where few combinations work well. It
    > seems so bad to me that a whitelist might be better than a blacklist.
    > That is probably overstating it, but NCQ performance is certainly a big
    > problem.

    Real world testing disagrees with you. NCQ has been enabled for a while
    now. We would have screaming hordes of users if the majority of
    configurations were problematic.


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