User guide

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Disk Mirroring (RAID 1)
Disk mirroring creates an identical twin for a selected disk by having the data simultaneously written to two
disks. This redundancy provides instantaneous protection from a single disk failure. If a read failure occurs on
one drive, the system reads the data from the other drive. RAID 1 sets are typically comprised of two drives,
and a third drive can be allocated as a spare in case one of the drives in the set fails. Additional drives can be
configured as part of a mirrored set, but without much added benefit. If the sizes of the disk segments are
different, the smallest disk segment will limit the overall size of the RAID Group.
Block 0
Block 1
Block 2
Block 3
Block 0
Block 1
Block 2
Block 3
Disk Mirroring and Striping (RAID 10)
RAID 10 combines the features of both RAID 0 and RAID 1. Performance is provided through the use of
Striping (RAID 0), while adding the fault tolerance of Mirroring (RAID 1). The implementation of RAID 10
requires four drives. The drives are assigned as two sets of mirrored pairs.
The data is written to RAID Group A, which is mirrored (RAID 1) and provides data redundancy. Alternating
blocks of data are then striped across another RAID 1 mirrored set, shown as Set B in the figure above. This
provides improved speed.
Under certain circumstances, a RAID 10 set can sustain multiple simultaneous drive failures.
Parity RAID (RAID 5)
Parity RAID, or RAID 5, adds fault tolerance to Disk Striping by including parity information with the data.
Parity RAID dedicates the equivalent of one disk for storing parity stripes. The data and parity information is
arranged on the disk array so that parity is written to different disks. There are at least 3 members to a Parity
RAID set. The following example illustrates how the parity is rotated from disk to disk.
Parity RAID uses less capacity for protection and is the preferred method to reduce the cost per megabyte for
larger installations. Mirroring requires 100% increase in capacity to protect the data whereas the above
example using three hard drives only requires a 50% increase. The additional required capacity decreases as
the number of disks in the group increases (i.e., 33% for four drives or 25% for five drives).