HP Surestore Disk Array 12h - And FC60 Advanced User's Manual page 54

more disks. For highest availability, the disks in a RAID 3 LUN must be in different
enclosures.
If a disk fails or becomes inaccessible, the disk array can dynamically reconstruct all user
data from the data and parity information on the remaining disks. When a failed disk is
replaced, the disk array automatically rebuilds the contents of the failed disk on the new
disk. The rebuilt LUN contains an exact replica of the information it would have contained
had the disk not failed.
Until a failed disk is replaced (or a rebuild on a global hot spare is completed), the LUN
operates in degraded mode. The LUN must now use the data and parity on the remaining
disks to recreate the content of the failed disk, which reduces performance. In addition,
while in degraded mode, the LUN is susceptible to the failure of the second disk. If a
second disk in the LUN fails while in degraded mode, parity can no longer be used and all
data on the LUN becomes inaccessible.
Figure 21 illustrates the distribution of user and parity data in a five-disk RAID 3 LUN. The
the stripe segment size is 8 blocks, and the stripe size is 40 blocks (8 blocks times 5 disks).
The disk block addresses in the stripe proceed sequentially from the first disk to the
second, third, and fourth, then back to the first, and so on.
Figure 21 LUN
RAID 3
Disk Array High Availability Features
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