Why Use RAID-1 Mirror?

A RAID-1 mirror set is a fault-tolerant flavor of RAID that can be understand as identical copies. The user sees one volume and icon, but there are two or more drives, each an identical copy

Typically a RAID-1 mirror uses dual drives, but a mirror can be made of 2/3/4/5/.. drives. RAID-1 offers fault tolerance by using two or more drives as exact copies:

• Little or no difference in performance over a single drive for writes, possible modest gains for reads.
• Capacity remains that of a single drive.
• Tolerates the failure of all but one drives in each mirror set. A dual-drive mirror can tolerate one drive failure, a 3-drive mirror can tolerate a 2-drive failure, etc.
• Software RAID-1 increases use of bandwidth for writes (double usage for a 2-drive mirror, triple usage for 3-drive mirror, etc).

Users looking for maximum fault tolerance can create a 3-drive or even 4-drive or more RAID-1 mirror. There is no performance gain to be had, except a modest one for readds.

Shown below to emphasize the point is an 8-drive RAID-1 mirror, which would allow an incredible 7 drive failures! That’s 8 X 5TB as shown, with capacity of 5TB, since there are 8 identical copies on 8 drives.

Creating a RAID volume in SoftRAID

Whereas Disk Utility simply kills an out of sync mirror drive, SoftRAID has a smart rebuild feature which can even be used for a backup-type purpose (pull a drive, then restore later):

Smart mirror rebuild notification