On 28/06/10 11:44, Chris G wrote:
A quick Google suggests that a 25Mb second transfer rate, reading one drive and writing to another, isn't too bad at all.
25Mb a second would be terrible. However 25MB a second would be ok for what might be a non sequential read and then write back onto another disk. Modern consumer grade drives can generally manage about 100MB/s on a sequential read in optimum circumstances.
It also depends on how your SATA ports are configured in the BIOS. Legacy or non AHCI mode usually makes them fall back to master/slave emulation which means that some SATA ports will share bus bandwidth, effectively halving the access speed if you are writing two and from disks on sequential channels.
You can determine this by running multiple copies of hdparm -tT /dev/sd(x). If you are in AHCI mode then you should get pretty much the same results running it on multiple drives at the same time as when running it on a single drive. If you are in Legacy mode you may well find that the performance drops massively.
BTW in case it isn't obvious this totally kills Linux MD array performance as well so it's worth checking on any machine.