Mirrored drives (RAID1) are, as you might guess, two drives that always have the same data on them. This is typically done on any critical server or system for the operating system. If one drive fails, the system can continue running on the other until it is replaced. The disadvantages are that you have to buy twice as many drives and that writing files is slower -- anytime you write a file, it has to be written to both drives. You do see a moderate increase in read-speed since the drives can alternate.

Mirroring is the only type of redundancy that doesn’t require parity calculations.