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    SubjectRe: Linux SATA RAID FAQ
    Alan Cox wrote:

    >Be cautious what you measure. One of he problems until you reach PCI-X
    >is PCI bandwidth. Thus software md5 can look good but the moment its
    >combined with other PCI activity goes down the pan entirely.
    >
    >
    Right, which is why you'd think hardware based RAID would fair better,
    the parity information or mirror'd writes would only require one
    transfer/transaction across the PCI bus. However, when benchmarking a
    3Ware 4-port ide raid controller (7000) we saw 40-50MB/sec read/write on
    4x160GB drives raid5. With md raid5 (same controller, HDs, FS, etc) we
    saw 100MB/sec read, 60MB/sec write. This was using max 15% CPU on a
    2.4GHz Pentium4, with a 32bit/33MHz PCI bus.

    >
    >
    >>When the libata Marvell drivers come out, you'll have a cheap upgrade
    >>path for PCI-X boards if you want fast md raid:
    >>
    >>
    >
    >Agreed. PCI-X will change a lot of this for boxes that are not very
    >cpu/memory limited.
    >
    >
    From the testing I've done, interconnect bandwidth has always been the
    limiting factor for the md driver. Using cheap ($~230) PCI-X
    motherboards--http://www.supermicro.com/products/motherboard/P4/E7210/P4SCT+.cfm--with
    dedicated gigE channels, you can make high density/price and
    performance/price NAS type appliances with no bottlenecks. For <$3K you
    can build a 2TB NAS server that'll keep a 250MB/s gigE link saturated.

    -ryan
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