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Chapter 7: Technology Background
181
RAID 1 – Mirror
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. With RAID 1, if one drive fails or has errors, 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.
Figure 2. RAID 1 Mirrors identical data to two drives
Due to the data redundancy of mirroring, the drive capacity of the disk array is
only the size of the smallest drive. For example, two 100GB 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 drives of different
capacities are used, there will also be unused capacity on the larger drive.
RAID 1 arrays use two physical drives. You can create multiple RAID 1 disk
arrays on the same Promise product.
Recommended applications: Accounting, Payroll, Financial, other applications
requiring very high availability.
Data Mirror
Disk Drives