So now we know emhttp doesn't parse /dev for devices, because I used mknod to create /dev/hda pointing to /dev/xvda (pretty much a low-level symlink) and the webGUI still ignored it.
Also, I got an email back from limetech specifically stating they only support /dev/hdX and /dev/sdX. I emailed back to see if I could get a workaround.
I'm pretty sure hdparm and ACPI in general doesn't work in XEN, since all the drives are emulated, even in PV mode. I'd rather just get it working first .
Speaking of which, I have successfully deployed unRAID in a Slackware 13.1 HVM domU running on a Debian Lenny (5.0) domU. I'm currently rebuilding the parity, so I'll test read/write speeds sometime over the weekend. I'm pretty certain I'll see at least a 50% hit from native, if not 75%, but speed really isn't an issue for me at the moment.
Hopefully support for /dev/xvdX is easily patched in, and we'll see it in a release soon.
EDIT: Parity is being build at about 21 MB/s, which isn't that bad for write speeds (80-100MB/s normal, unraid writes 4x for each normal write). It seems almost native?