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LaCie 5big Network
RAID and Disks
User Manual
page 52
5.3. RAID 5
In RAID 5, data is striped across all disks (minimum of three) and a
parity block for each data block (P in
Fig. 85
) 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.
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 archiving and for people
who need performance and constant access to their data, like video
editors.
How RAID 5 Capacity Is Calculated
Each disk in a RAID 5system should have the same capacity.
Storage capacity in a RAID level 5 configuration is calculated by
subtracting the number of drives by one and multiplying by the disk
capacity, or
C = (n-1)*d
where:
C = available capacity
n = number of disks
d = disk capacity
For example, in a RAID 5 array with four drives each with a capac-
ity of 1000GB, the total capacity of the array would be 3000GB:
C = (4-1)*1000
RAID 5
Disk 2
A2
B2
D1
Cp
Disk 1
A1
B1
C1
D1
Ep
A2
B2
C2
Dp
E1
Disk 3
A3
C2
D2
Bp
Disk 4
B3
C3
D3
Disk 5
A3
B3
Cp
D2
E2
A4
Bp
C3
D3
E3
Ap
B4
C4
D4
E4
Fig. 85