I've yet to see SCSI DMA turned off for anything. I suppose it might've been on an ancient ISA controller for a CD, but that would be about it.I was under the impression that the scsi raid controller (adaptec 2120S) did DMA by default?
I ran hdparm while in X with a lot of programs running:
[snip]
I seem to get quit[e] different readings each time I run hdparm.
Well you would. hdparm -t measures the maximum sustained data transfer rate and if you have other programs using the disc then the occasional seek will badly skew the reported result. I'm told that hdparm is not a good way to measure the performance of SCSI discs and that tiobench (http://sf.net/projects/tiobench/) is a lot more useful. Certainly on my own system, hdparm consistenly reported about 50Mb/s from a SCSI disk, but when moving some big files around a couple of days ago, gkrellm reported a transfer rate of 190Mb/s for one brief interval.
jch
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