Promise Technology FASTTRAK SX4300 - Manual (Page 43)

Chapter 5: Concepts
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RAID 1 – Mirror
When a logical drive is mirrored, identical data is written to a pair of disk 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.
With RAID 1, if one disk drive fails or has errors, the other mirrored disk drive
continues to function. This is called Fault Tolerance. Moreover, if a spare disk
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.
Figure 2. RAID 1 Mirrors identical data to two drives
Due to the data redundancy of mirroring, the capacity of the logical drive is only
the size of the smallest disk drive. For example, two 100GB disk drives which
have a combined capacity of 200GB instead would have 100GB of usable
storage when set up in a mirrored logical drive. Similar to RAID 0 striping, if disk
drives of different capacities are used, there will also be unused capacity on the
larger drive.
RAID 1 logical drives on the FastTrak SX Series consist of two disk drives.
When two drives do not offer sufficient data capacity, consider a RAID 10 logical
drive, see page 39.
Data Mirror
Disk Drives