Re: 20TB Storage System (fsck????)

From: Chuck Swiger (cswiger_at_mac.com)
Date: 09/03/03

  • Next message: Brooks Davis: "Re: 20TB Storage System (fsck????)" 
    Date: Wed, 03 Sep 2003 15:37:35 -0400

    Piotr KUCHARSKI wrote:
    > On Wed, Sep 03, 2003 at 02:37:50PM -0400, Chuck Swiger wrote:
    >>I'm wary of the write-performance hit from putting too many drives wide
    >>in a RAID-5 (or -5,0) configuration.
    >
    > How many is "too many"?

    At one point, the advice used to be to use between four and seven disks for a
    RAID-5 volume. For example, the Apple XServe RAID box has 14 bays, but Apple
    seems to recommend configuring it as two 7-drive RAID-5 volumes, rather than a
    single 14-drive-wide RAID-5 volume.

    > Or, rather, what are write-performance penalties
    > when using sixteen disks in one hw raid5 set? (With two raid volumes,
    > 2TB and 1.75TB available for OS.)

    Find yourself a buncha small files-- a CVS repository, or /usr/ports will do,
    and compare write performance to a single drive versus RAID-5. Basicly, you get
    all of the drives in the RAID-set scribbling away at a fraction of the write
    speed of a single drive, yes? Three disk transactions per write, versus one?

    Also note that all this disk activity requires three times the I/O bandwidth,
    interrupts, and assorted overhead. If the OP has hardware RAID which is
    designed to support a wide array, OK, but setting up a too-wide a RAID-5 array
    means that things like the system bus may bottleneck performance, rather than
    the drives.

    Normally, disk I/O speed is the limiting factor, and your bus and memory are
    sitting around waiting for the DMA to complete (well, being used by the CPU to
    run other processes). Let's put it this way, things don't go faster when the
    drives are waiting for the bus to become available, rather than vice-versa. :-) 

    -- 
-Chuck
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  • Next message: Brooks Davis: "Re: 20TB Storage System (fsck????)"

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