We recommend that your mirror volumes have 3 disks. This allows you to have two disks connected to your Mac at all times, a mirror pair. They can be two internal disks in a Mac Pro, or Mac Mini Server or a pair of external eSATA, FireWire or USB disks. The two disks act as a traditional mirror, if one disk fails while you are working, the second disk takes over and allows you to continue working.
The third disk in your mirror volume should be an external disk which is normally stored in another building. This disk should be connected up to the Mac with the other two disks in the mirror periodically so it can be rebuild and get updated with the most recent volume files. This third disk provides you with a disaster recovery mechanism. If your Mac gets stolen or the building your Mac is in burns down, you will still have the third mirror disk which you can use. At SoftRAID, we rebuild to our third mirror disk every 2 weeks. That way, the most we will ever lose is 2 weeks worth of work.
Fast Mirror Rebuilds happen so quickly that users often wonder whether all their mirror volumes contain the same data. If your 2 terabyte volume rebuilds in 10 - 20 minutes, you might think that some parts of the volume weren't rebuilt correctly.
We added the volume validate function to SoftRAID for just this purpose. When validating a mirror volume, SoftRAID checks every sector from each of the disks in the volume to ensure the data is correct. We used the volume validate feature extensively when we were testing the Fast Mirror Rebuild code to ensure that it was rebuilding mirror volumes correctly. Read more about validating mirror volumes in the section below.
SoftRAID validates a mirror volume by reading every sector from each of the disks in the volume, and comparing them to ensure they all have the correct data—ie the mirrored data is identical to the data it's mirroring. If a mismatch is found, SoftRAID will update all blocks on the volume's disks with the correct data. The SoftRAID application displays a count of the number of blocks updated whenever you validate a mirror volume. Since each disk can be used for more than one volume, only the parts of the disk that contain the actual volume being validated are checked.
If a mirror volume has been validated successfully with SoftRAID, you are guaranteed that all the disks for that mirror volume contain identical data.
You can read more about the validation process and how it works for other volume types here