User manual

LaCie 4big Quadra • Design by Neil Poul ton RAID
User Manual page 18
RAID 5 and RAID 5+Spare3.1.5.
RAID 5 combines the striping of RAID 0 with data redundancy in
an array with a minimum of three disks.
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.
RAID 5+Spare
Disk 1 Disk 2 Disk 3 Disk 4
Spare
A2
D2
C1
Bp
A1
B1
D1
Cp C2
B2
Ap
Dp
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
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.