Promise Technology VTrak 15110 - Manual (Page 217)

Chapter 7: Technology Background
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RAID 1 – Mirroring
When a disk array 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 disk array 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 disk array. 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 disk arrays on VTrak consist of two disk drives.
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2
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4
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2
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Data Mirror
Disk Drives