HP StorageWorks XP48 - Disk Array Owner's Manual page 105

Ready to accept
I/O requests
DKC
Spare
Disk 1 Disk 2 Disk 3 Disk 4
disk
Ready to accept Ready to accept
I/O requests I/O requests
DKC (RAID5) DKC (RAID 0/1)
Spare Spare
Disk 1 Disk 2 Disk 3 Disk 4 Disk 1 Disk 2 Disk 3 Disk 4
disk disk
RAID 5 RAID 0/1
Figure 19. Dynamic Sparing and Correction
Dynamic sparing is a method of removing a drive from service if its
read/write errors exceed a certain threshold value. On normal read and
write operations, the array keeps track of the number of errors that occur. If
the error threshold is reached, the system considers that the drive is likely to
cause an unrecoverable error and automatically copies the data from that
drive to a spare disk. The dynamic sparing method used in the XP512 is the
same for both RAID0/1 and RAID5.
Correction copy is invoked when the system encounters an unrecoverable
error occurring on a drive. In RAID5, correction copy will regenerate the
original data for that drive using the data from the other two drives and the
parity data, and then the controller copies the regenerated data onto the
spare disk. In RAID0/1, correction copy just has to copy the data from the
mirrored disk onto the spare disk. After a failed disk has been replaced, the
data is moved back to its original location, and the spare disk is then
available as an active spare.
RAID Technology
105