User manual

WebPAM User Manual
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RAID 1 - Mirroring
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.
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Disk Drives
Data Mirror
Figure 106. RAID 1 Mirrors Identical Data to Two Drives
Due to the data redundancy of mirroring, the drive capacity of the 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 array. Similar to RAID 0 striping, if drives of different
capacities are used, there will also be unused capacity on the larger drive.
RAID 0+1 - Striping / Mirroring
Striping/mirroring combines both of the previous array types. It can increase
performance by reading and writing data in parallel while protecting data with
duplication. At least four drives are needed for RAID 0+1 to be installed. With a
four-drive disk array, drive pairs are striped together with one pair mirroring the
first pair.