LaCie 4big Quadra Enterprise Class
• D
esign
by
n
eil
P
oulton
RAID
User Manual
page 17
3.1.5. RAID 5 and RAID 5+Spare
RAID 5 combines the striping of RAID 0 with data redundancy in an
array with a minimum of three disks (
Fig. 12
).
Data is striped across all disks and a parity block (P) for each data
block is written on the same stripe. If one physical disk fails, the data
from the failed disk can be rebuilt onto a replacement disk. No
data is lost in the case of a single disk failure, but if a second disk
fails before data can be rebuilt to a replacement drive, all data in
the array will be lost.
In RAID 5+Spare
, one disk in the array is left empty. If a drive in
the array fails, the data from the failed disk is automatically rebuilt
to the empty or “spare” disk (
Fig. 13
).
Applications
RAID 5 combines data safety with efficient use of disk space.
Disk failure does not result in a service interruption because
data is read from parity blocks. RAID 5 is useful for people
who need performance and constant access to their data,
like video editors.
In RAID 5+Spare
disk failure does not require immedi-
ate attention because the system rebuilds itself using the hot
spare, but the failed disk should be replaced as soon as
possible.
RAID 5
Disk 1
Disk 2
Disk 3
Disk 4
Ap
B3
C3
D3
A3
C2
D2
Bp
A2
B2
D1
Cp
A1
B1
C1
Dp
RAID 5+Spare
Disk 1
Disk 2
Disk 3
Disk 4
Spare
A2
D2
C1
Bp
A1
B1
D1
Cp
C2
B2
Ap
Dp
Fig. 12
Fig. 13
