Promise Technology 66 Pro - Manual (Page 127)

SuperTrak66™ User's Manual Appendix A
A-3
Mirroring (RAID 1)
When a disk array is mirrored, identical data is written to a pair of drives, while
reads are performed in parallel. The reads are performed using elevator seek and
load balancing techniques where the workload is distributed in the most efficient
manner. Whichever drive is not busy and is positioned closer to the data will be
accessed first.
Under RAID 1, if one physical drive suffers a mechanical failure or sector error, the
other mirrored drive continues to function. This is called Fault Tolerance. Moreover,
if a spare drive is present, the spare drive will be used as the replacement drive and
data will begin to be mirrored to it from the remaining good drive.
RAID 1 (Mirroring)
Figure A2: RAID 1 mirrors identical data to two drives
Due to the data redundancy of mirroring, the drive capacity of the array is only the
size of the smallest drive. For example, two 1GB drives which have a combined
capacity of 2GB instead would have 1GB of usable storage when set up in a
mirrored array. Similar to RAID 0 striping, if drives of different capacities are used,
there will also be unused capacity on the larger drive.