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    SubjectRe: Performance Characteristics of All Linux RAIDs (mdadm/bonnie++)
    Justin Piszcz wrote:
    > On Wed, 28 May 2008, Chris Snook wrote:
    >> Justin Piszcz wrote:
    >>> Hardware:
    >>> 1. Utilized (6) 400 gigabyte sata hard drives.
    >>> 2. Everything is on PCI-e (965 chipset & a 2port sata card)
    >>> Used the following 'optimizations' for all tests.
    >>> # Set read-ahead.
    >>> echo "Setting read-ahead to 64 MiB for /dev/md3"
    >>> blockdev --setra 65536 /dev/md3
    >>> # Set stripe-cache_size for RAID5.
    >>> echo "Setting stripe_cache_size to 16 MiB for /dev/md3"
    >>> echo 16384 > /sys/block/md3/md/stripe_cache_size
    >>> # Disable NCQ on all disks.
    >>> echo "Disabling NCQ on all disks..."
    >>> for i in $DISKS
    >>> do
    >>> echo "Disabling NCQ on $i"
    >>> echo 1 > /sys/block/"$i"/device/queue_depth
    >>> done
    >> 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.

    -- Chris

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      Last update: 2008-05-28 21:25    [W:0.021 / U:9.036 seconds]
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