Promise Technology FastTrak SX Series Version 4.4 - Manual (Page 89)

Appendix A: RAID Concepts
85
Mirroring (RAID 1)
Writes duplicate data on to a pair of drives while reads are performed in parallel.
RAID 1 is fault tolerant because each drive of a mirrored pair is installed on
separate controller channels. If one of the mirrored drives suffers a mechanical
failure (for example, spindle failure) or does not respond, the remaining drive will
continue to function. This is called Fault Tolerance. If one drive has a physical
sector error, the mirrored drive will continue to function.
1
2
3
4
1
2
3
4
Disk Drives
Data Mirror
Figure 84. RAID 1 copies the data from one drive to the other
PAM will display an error in the array and recommend replacing the failed drive.
Users may choose to continue using their PC, however Promise recommends
replacing the failed drive as soon as possible.
Due to redundancy, the drive capacity of the array is half the total drive capacity.
For example, two 100 GB drives that have a combined capacity of 200 GB
would have 100 GB of usable storage. With drives of different capacities, there
may be unused capacity on the larger drive.
Using a Spare Drive – Under a RAID 1 setup, an extra hot spare drive. You can
attach a third drive to the FastTrak SX4000 series and S150 SX4 without
assigning it to the array. See Controller Rebuild Settings on page 59. Such a
drive will be activated to replace a failed drive that is part of a mirrored array. A
rebuild takes place automatically in the background to mirror the good drive data
on to the spare.
At a later time, the system can be powered off and the failed drive can be
physically removed and replaced. Spare drives must be the same or larger
capacity than the smallest array member.