User manual

SuperTrak SX6000™ User Manual Appendix C
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.
Figure 16 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 10GB drives which have a combined capacity of
20GB instead would have 10GB 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/Mirror
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 RAID0+1 to be installed. With a four-
drive disk array, drive pairs are striped together with one pair mirroring the first pair. The
data capacity is similar to a standard mirroring array, with half of the total storage
capacity dedicated for redundancy. An added plus for using RAID 0+1 is that, in many
situations, such an array offers double fault tolerance. Double fault tolerance may allow
your data array to continue to operate depending on which two drives fail.
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