HP Surestore Disk Array 12h and FC60 - Manual (Page 61)

Disk Array High Availability Features 61
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Global Hot Spare Disks
A global hot spare disk is reserved for use as a replacement disk if a data disk fails. Their
role is to provide hardware redundancy for the disks in the array. To achieve the highest
level of availability, it is recommended that one global hot spare disk be created for each
channel. A global hot spare can be used to replace any failed data disk within the array
regardless of what channel it is on.
When a disk fails, the disk array automatically begins rebuilding the failed disk’s data on an
available global hot spare. When all the data has been rebuilt on the global hot spare, the
LUN functions normally, using the global hot spare as a replacement for the failed disk. If a
global hot spare is not available, data is still accessible using the redundant data
maintained by the LUN.
When the failed disk is replaced, all data is copied from the former global hot spare onto
the replacement disk. When the copy is complete, the former global hot spare is returned to
the global hot spare disk group and is again available as protection against another disk
failure.
If a failed disk is replaced while data is being rebuilt on the global hot spare, the rebuild
process continues until complete. When all data is rebuilt on the global hot spare, it is then
copied to the replacement disk.
Global hot spares are an essential component for maintaining data availability. A global hot
spare reduces the risk of a second disk failure and restores the disk array’s performance,
which may be degraded while the LUN is forced to recreate data from parity. The use of
multiple global hot spares may be desirable in environments where data availability is
crucial. Multiple global hot spares ensure that data remains accessible even if multiple
disks fail.
Rebuilding Data
The rebuild process occurs any time a disk failure occurs. It uses the existing data and
parity or mirror disk to rebuild the data that was on the failed disk. Because it is competing
with host I/Os for disk array resources, a rebuild may affect disk array performance. The
effect on performance is controlled by the rebuild priority settings. These settings
determines how the disk array divides resources between the rebuild and host I/Os.