Mirror Disks After Install
Disk mirroring, where data is written to two disks simultaneously, is a great low cost method to protect against single-disk failure and improve read-intensive performance. Apple's Disk Utility provides an easy way to set two disks up as a RAID mirror at installation, but once the operating system has been installed OS X Tiger can't mirror an existing drive without reformatting and reinstalling. Fortunately, the command line offers an easy method to build a mirrored RAID array from an existing volume.
Like most command line operations run as an administrator, this process has the potential to destroy all the data on your machine, so make a backup of the drive you're going to mirror and confirm the backup is both restorable and bootable. Next, if you're mirroring your existing startup disk, you'll need to boot off an installation DVD or external drive. When you're ready to proceed, type the following into the Terminal:
diskutil list
This will produce a listing of the attached disks, along with their types, names, sizes, and identifiers, like so:
/dev/disk1
#: type name size identifier
0: Apple_partition_scheme *465.8 GB disk1
1: Apple_partition_map 31.5 KB disk1s1
2: Apple_HFS Server Disk 465.6 GB disk1s2
Use the name of the volume you wish to mirror to find it's identifier. If your machine only has one internal hard drive, for instance, the identifier of your boot volume will most likely be disk1s2, the second slice (after the partition scheme and map) of disk 1. Once you've determined which disk you're working with, type the following, replacing IDENTIFIER appropriately:
sudo diskutil enableRAID mirror IDENTIFIER
If all goes well, the disk will unmount, reappearing in the Finder a moment later along with "The disk has been converted into a RAID" reported in the Terminal. At this point you can add the additional disk you wish to use in the mirrored RAID array (taking care that it's the same size as the original), then reboot the machine off the original drive.
Now when when you run diskutil list from the Terminal you should see two new listings, one for the new virtual RAID array (the one without slice information) and the other for the additional physical disk you installed.
/dev/disk2
#: type name size identifier
0: Apple_HFS Server Disk *465.6 GB disk2
/dev/disk3
#: type name size identifier
0: GUID_partition_scheme *465.6 GB disk3
1: EFI 200.0 MB disk3s1
2: Apple_HFS New Disk 465.4 GB disk3s2
Now with the necessary identifier information, you can assign the physical disk to the mirrored RAID array, replacing RAIDARRAY with the RAID volume (in our example disk2) and NEWDISK with the hard drive being added (here disk3):
sudo diskutil repairMirror RAIDARRAY NEWDISK &
How long the RAID mirror takes to build depends on how much data is on the original volume, but the ampersand at the end of the command tells it to run in the background. When the process is completed you'll have two fully mirrored and redundant disks, built from your existing installation without ever having to erase the original volume.
UPDATE: A few months after this article was written, Apple broke diskutil and by extension Disk Utility, making it impossible to rebuild a mirror booted from the degraded volume on 10.4.10 and above. Currently, the work around is to preform all steps of the operation off a boot DVD, which unfortunately means keeping your server offline while the RAID mirror rebuilds.
