HP Surestore Disk Array 12h - And FC60, Advanced User'S Manual, Page: 48 / 466

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Disk Array High Availability Features
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 LUNs. A RAID 1 LUN consists of exactly
two disks: a primary disk and a mirror disk. A RAID 0/1 LUN 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 LUN 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 LUN to reconstruct the data on the failed disk. Data parity is used for RAID 3 and RAID
5 LUNs.
To ensure high availability, each disk in the LUN should be in a separate enclosure. Parity
cannot be used to reconstruct data if more than one disk in the LUN 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 LUN. 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 LUN. 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.