Mdadm

So today I played a bit with some big cheap disks. I have 3 fairly old desktops each with 4*1TB disks, all exported via ATA over Ethernet to a much more modern sort of disk head. Basically it’s a “build a large store on half a shoe string” project. I’ve not quite got the network side of it sorted yet. Currently the disk head’s gigabit card is being saturated. On a single disk node each disk can do about 80M read sustained. If all 4 are read at the same time it goes down to about 50M, which is 200M in total. Which seems quite amazing for a Pentium 4 3.0Ghz. Also this seems a bit high as the PCI bus can only do 133M, I’m guessing that the onboard sata ports are somehow separated from the extra pci sata card I added. Interestingly one disk node can sustain 80M read from disk to the disk head. Again this backs down to 30M if all 4 disks are read, that’s 120M total, no surprise this is saturating the gigabit link. So the major bottleneck is the disk head. Currently max raid sync speed is 10M, ie 120M total for 12 disks. Ideally 3 nics in the disk head would be best, but then there is no way for the data to get to the disk head.