LaCie 5big Network
RAID and Disks
User Manual
page 53
5.4. RAID 5+Spare
RAID 5+Spare, is a RAID 5 array in which one disk is used as spare
to rebuild the system as soon as a disk fails. At least four disks are
required (
Fig. 86
).
If one physical disk fails, the data remains available because it is
read from the parity blocks. Data from a failed disk is rebuilt onto
the hot spare disk. When a failed disk is replaced, the replacement
becomes the new hot spare. No data is lost in the case of a single
disk failure, but if a second disk fails before the system can rebuild
data to the hot spare, all data in the array will be lost.
Applications
RAID 5+ Spare has good data safety but disk space is limited by
the presence of the hot spare which is not used until one of the
other disks fails. RAID 5+ is useful in settings where users must have
constant access to data because disk failure does not result in a ser-
vice interruption. Disk failure does not require immediate attention
because the system rebuilds itself using the hot spare, but the failed
disk should be replaced as soon as possible.
How RAID 5+Space Capacity Is Calculated
Each disk in a RAID 5+Spare system should have the same capacity.
Storage capacity in a RAID 5+Spare configuration is calculated by
subtracting the number of drives by two and multiplying by the disk
capacity, or
C = (n-2)*d
where:
C = available capacity
n = number of disks
d = disk capacity
For example, in a RAID 5+Spare array with four drives each with
a capacity of 1000GB, the total capacity of the array would be
2000GB:
C = (4-2)*1000
RAID 5+Spare
Spare
Disk 5
Disk 4
Ap
B3
C3
D3
Disk 3
A3
C2
D2
Bp
Disk 2
A2
B2
D1
Cp
Disk 1
A1
B1
C1
Dp
Fig. 86
