LaCie 4big Quadra Enterprise Class
• D
esign
by
n
eil
P
oulton
RAID
User Manual
page 16
3.1.4. RAID 3 and RAID 3+Spare
RAID 3 uses byte-level striping with a dedicated parity disk (Disk 4 in
Fig. 10
and
Fig. 11
) so that one volume mounts on the computer.
A RAID 3 array can tolerate a single disk failure without data loss.
If one physical disk fails, the data from the failed disk can be rebuilt
onto a replacement disk. If a second disk fails before data can be
rebuilt to a replacement drive, all data in the array will be lost.
In RAID 3+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. 11
).
Applications
RAID 3 provides good data safety for environments where
long, sequential files are being read, such as video files.
Disk failure does not result in a service interruption because
data is read from parity blocks. RAID 3 is useful for people
who need performance and constant access to their data,
like video editors. RAID 3 is not recommended for intensive
use with nonsequential files because random read perfor-
mance is hampered by the dedicated parity disk
In RAID 3+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.
A1
A4
B1
B4
C1
C4
A2
A5
B2
B5
C2
C5
A3
A6
B3
B6
C3
C6
Ap(1-3)
Ap(4-6)
Bp(1-3)
Bp(4-6)
Cp(1-3)
Cp(4-6)
RAID 3
Disk 1
Disk 2
Disk 3
Disk 4
RAID 3+ Spare
Disk 1
Disk 2
Disk 3
Disk 4
A1
A3
B1
B3
C1
C3
A2
A3
B2
B4
C2
C4
Ap(1-2)
Ap(3-4)
Bp(1-2)
Bp(3-4)
Cp(1-2)
Cp(3-4)
Spare
Fig. 10
Fig. 11
