Server Fault is a question and answer site for system and network administrators. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I'm installing FreeBSD 8.0 on a server that has hardware SATA Raid.

I'm just wondering. What is the difference between these devices.

i.e. ar0, da0, ad4, ad5 I take it that ad4 & 5 are my two disks. Somehow the OS can see them individually even though it's one logical mirrored drive.

Should I be installing it onto ar0 or one of the adX disks. What is da0? it's smaller than the others. ar0 is not some kind of software raid device is it?

Just want to make sure I don't mess this up right from the get go.

share|improve this question

It's hard to say without knowing the system configuration (controller type, etc.) but you probably want to install onto the arN device -- see (Hardware RAID - about 3/4 of the way down) & the ataraid(4) manpage.

See also the gmirror(8) manpage & various bits of documentation on using that -- it's a bit more initial work, but I find it better documented than the ataraid stuff...

share|improve this answer
ar0. da0 - the flash disk plugged into the server. ad4/5 the disks. Not sure what would happen if I install on either of those. After all it's hardware raided. Will it still mirror? – Matt Mar 3 '10 at 19:57
Most of the SATA RAID stuff is actually software RAID (which is why you're getting an ar0 device) -- If you have what FreeBSD considers a "real RAID controller" you'd have a different device name. I'm honestly not sure what would happen if you have the SATA RAID option enabled in BIOS and you write directly to one of the underlying adN disks but my gut says "bad things", which is why you'd want to write to the ar0 device (or switch off the BIOS RAID and do it with gmirror or similar :-) – voretaq7 Mar 3 '10 at 20:36
Commonly known as fakeraid -… – Andrew Jun 18 '10 at 4:58

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.