SATA READ_DMA timeouts - SOLVED?

I've seen a number of people having DMA troubles with SATA disks on FreeBSD6 and FreeBSD7, and I'm in the same boat. A while back I posted looking for help but none could really be found. Today I finally got to the bottom of things (at least so far).

Hardware incompatibility.

According to http://en.wikipedia.org/wiki/SATA#SATA_1.5_Gbit.2Fs_and_SATA_3_Gbit.2Fs there is an upward compatibility problem between a number of VIA and SiS chipsets and SATA300 disks. I happen to have one of those controllers (SiS964) and a pair of WD1600AAJS disks, which are SATA300 disks.

I ripped apart my machine, and sure enough I had a jumper on each disk labelled 'OPT1', which is documented to force SATA150 operation.

I've since cold booted, warm booted, and booted after a power interruption with no READ_DMA timeouts on these disks. I think this solved the problem in my case.
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