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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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