Messages in this thread | | | Date | Sat, 14 Aug 2004 16:28:33 -0600 | From | "J. Ryan Earl" <> | Subject | Re: Linux SATA RAID FAQ |
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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.
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