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    SubjectRe: Performance Characteristics of All Linux RAIDs (mdadm/bonnie++)


    On Wed, 28 May 2008, Chris Snook wrote:

    > Justin Piszcz wrote:
    >>
    >>
    >> On Wed, 28 May 2008, Chris Snook wrote:
    >>
    >>> Justin Piszcz wrote:
    >>>> Hardware:
    >>>>
    >>> Given that one of the greatest benefits of NCQ/TCQ is with parity RAID,
    >>> I'd be fascinated to see how enabling NCQ changes your results. Of
    >>> course, you'd want to use a single SATA controller with a known good NCQ
    >>> implementation, and hard drives known to not do stupid things like disable
    >>> readahead when NCQ is enabled.
    >> Only/usually on multi-threaded jobs/tasks, yes?
    >
    > Generally, yes, but there's caching and readahead at various layers in
    > software that can expose the benefit on certain single-threaded workloads as
    > well.
    >
    >> Also, I turn off NCQ on all of my hosts that has it enabled by default
    >> because
    >> there are many bugs that occur when NCQ is on, they are working on it in
    >> the
    >> libata layer but IMO it is not safe at all for running SATA disks w/NCQ as
    >> with it on I have seen drives drop out of the array (with it off, no
    >> problems).
    >>
    >
    > Are you using SATA drives with RAID-optimized firmware? Most SATA
    > manufacturers have variants of their drives for a few dollars more that have
    > firmware that provides bounded latency for error recovery operations, for
    > precisely this reason.
    I see--however, as I understood it there were bugs utilizing NCQ in libata?

    But FYI--
    In this test, they were regular SATA drives, not special raid-ones (RE2,etc).

    Thanks for the info!

    Justin.



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