Data Parity - HP Surestore Disk Array 12h - And FC60 Service Manual

The disk array uses hardware mirroring, in which the disk array automatically
synchronizes the two disk images, without user or operating system involvement. This is
unlike the software mirroring, in which the host operating system software (for example,
LVM) synchronizes the disk images.
Disk mirroring is used by RAID 1 and RAID 0/1 volume groups. A RAID 1 volume group
consists of exactly two disks: a primary disk and a mirror disk. A RAID 0/1 volume group
consists of an even number of disks, half of which are primary disks and the other half are
mirror disks. If a disk fails or becomes inaccessible, the remaining disk of the mirrored pair
provides uninterrupted data access. After a failed disk is replaced, the disk array
automatically rebuilds a copy of the data from its companion disk. To protect mirrored data
from a channel or internal bus failure, each disk in the volume group should be in a
different enclosure.

Data Parity

Data parity is a second technique used to achieve data redundancy. If a disk fails or
becomes inaccessible, the parity data can be combined with data on the remaining disks in
the volume group to reconstruct the data on the failed disk. Data parity is used for RAID 3
and RAID 5 volume groups.
To ensure high availability, each disk in the volume group should be in a separate
enclosure. Parity cannot be used to reconstruct data if more than one disk in the volume
group is unavailable.
Parity is calculated on each write I/O by doing a serial binary exclusive OR (XOR) of the
data segments in the stripe written to the data disks in the volume group. The exclusive OR
algorithm requires an even number of binary 1s to create a result of 0.
Figure 17 illustrates the process for calculating parity on a five-disk volume group. The
data written on the first disk is "XOR'd" with the data written on the second disk. The result
is "XOR'd" with the data on the third disk, which is "XOR'd" with the data on the fourth
disk. The result, which is the parity, is written to the fifth disk. If any bit changes state, the
parity also changes to maintain a result of 0.
Disk Array High Availability Features
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