Raid 1 - Mirror - Promise Technology FastTrak TX4650 User Manual

Chapter 6: Technology
RAID 1 – Mirror
When a logical drive is mirrored, identical data is written to a pair of physical
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 physical drive fails or has errors, the other mirrored physical
drive continues to function. This is called Fault Tolerance. Moreover, if a spare
physical 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.
Data Mirror
Physical Drives
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 physical drive. For example, two 100 GB physical drives
which have a combined capacity of 200 GB instead would have 100 GB of usable
storage when set up in a mirrored logical drive. Similar to RAID 0 striping, if
physical drives of different capacities are used, there will also be unused capacity
on the larger drive.
RAID 1 logical drives on the FastTrak TX Series consist of two physical drives.
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