Raid 5+Spare - LaCie 5big network User Manual

LaCie 5big network
User Manual
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.
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 re-
placement 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 service 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 ca-
pacity.
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
A1 A2 A3
B1 B2 Bp
C1 Cp C2
Dp D1 D2
Disk 1 Disk 2 Disk 3
RAID and Disks
page 52
Ap
B3
Spare
C3
D3
Disk 4 Disk 5