Parity-Protected Disk
Parity-protected disk subsystems are a more cost-effective way
to protect from disk drivefailures than disk mirroring. The most popular
form of parity-protected disk storage is RAID 5, which effectively adds an extra
“parity” drive to a set of “data drives” to allow the total set of drives to
continue to deliver data even when a single drive fails. For example, a 4+1 RAID
5 set would have four data drives and a parity drive. In this example, only 25
percent more disk storage capacity is required to protect the data drives. In
contrast, disk mirroring requires 100 percent more disk storage capacity to
protect data.
Some RAID 5 disk sets may also include another “hot-spare” disk
drive that is used to immediately start the rebuild process when any of the
other RAID 5 drives fail. Alternatively, instead of a hot spare, when a disk
drive fails in the set, it can be removed and replaced with a new disk drive,
but this can lengthen the time to bring the RAID 5 disk set back to full
protection status.
If two drives fail simultaneously in a RAID 5 disk set or a
second drive fails while the first drive is being rebuilt using the spare, all
data is lost and must be restored, typically from weekend and nightly backup
tapes. With disk drive capacities growing even beyond the 500GB range, the time
it takes to completely rebuild a failed drive can take many hours, and even days
if performed as a background operation. This extended rebuild period creates a
greater probability that a second drive could fail causing all data in the RAID
5 array to be lost.
RAID 6 is becoming increasingly popular as an improved
parity-protection option for magnetic disk subsystems. For slightly more
physical disk storage capacity than RAID 5, a RAID 6 array can continue to
operate even when up to two drives in the array have failed. With RAID 6, it
takes the equivalent of two drives per array to maintain parity data.
Like mirrored disk systems, RAID 5 and RAID 6 disk subsystems do
not guard against deleted or corrupted files and therefore require nightly
backups in order to keep a day-to-day history for recovery from deleted or
corrupted files.
There are many other configurations of parity protected RAID
disk storage. For instance, RAID 51 provides two RAID 5 arrays that are
synchronously or asynchronously mirrored either locally or at two different
sites.