This may be of more interest to Mac sysadmins than to the rest of my readers, but it was really, really, really interesting to me, in that kind of “OMG did we just lose everything” way that software so often is.
You can boot OS X from a software RAID (a virtual RAID device configured using the OS’s software, rather than hardware RAID).
If you are using an Intel Mac, you must format the disks using normal GUID partition tables, then use the partitions on those disks in your RAID. Otherwise, there’s no GUID partition table and the machine can’t boot.
This is not immediately obvious.
While I’m at it, be sure to enable group/uid management on a disk (“vsdbutil -a /Volumes/Raid”) before trying to ditto a root filesystem to it.
Here’s hoping this saves you the several hours it just cost me.