Re: disk write barriers

From: Lowell Gilbert (freebsd-questions-local_at_be-well.ilk.org)
Date: 07/07/05

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    To: Matthias Buelow <mkb@incubus.de>
To: freebsd-questions@freebsd.org
Date: 07 Jul 2005 09:57:08 -0400

    Matthias Buelow <mkb@incubus.de> writes:

    > Now my question is: Does FreeBSD also implements these barriers, or an
    > equivalent mechanism, as claimed on the above URL?

    Do you have a background in OS design? It affects the answer, because
    you seem to be referring to access barriers and disk cache flushes
    interchangeably, which doesn't make sense, especially on
    multiprocessor systems.

    > If it does, why then
    > the frequent advice to disable write-back caching? Or is that only for
    > the couple drives that ignore any flush cache/disable wb commands?

    The problem with caching has nothing to do with flushing the cache; if
    you flush the cache often, there's no advantage to using it anyway.
    The whole speedup from using on-disk caching comes from the fact that
    the drive reorders the writes at will, and lies to the operating
    system by saying the writes are done when they aren't. Among other
    obvious problems, this negates the careful ordering calculated by
    softupdates.

    > I
    > know my drive doesn't ignore that command (since performance drops
    > sharply, when I disable the cache), so I would be on the safe side.

    This doesn't follow. Just because you know that your drive supports
    disabling the cache does not mean that it is safe to do so.

    > Would it be possible to get an authoritative statement from a FreeBSD
    > developer here on that matter?

    Could depend on how much you're willing to pay them.
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