Raid 5; Hot-Spare; Table 41 Raid 5 - ZyXEL Communications NSA-2401 User Manual

Chapter 6 Storage Screens
Typical applications for RAID 10 are those requiring both high performance and reliability
such as enterprise servers and high-end moderate-sized database systems. RAID 10 is often
used in place of RAID 1 or RAID 5 by those requiring higher performance. It may be used
instead of RAID 1 for applications requiring more capacity.

6.9.6 RAID 5

RAID 5 provides the best balance of capacity and performance while providing data
redundancy. It provides redundancy by striping data across three disks and keeps the parity
information (AP) on the fourth disk (in each stripe). In case of disk failure, data can be
recovered from the surviving disks using the parity information. When you replace the failed
disk, the reconstructed data is written onto the new disk. Resynchronize the array to have it
return to its original state. The following example shows data stripped across three disks (A1
to A3 in the first strip for example) with parity information (AP) on the fourth disk

Table 41 RAID 5

A1
B1
C1
DP
DISK 1
The capacity of a RAID 5 array is the smallest disk in the RAID set multiplied by one less than
the number of disks in the RAID set. For example, if you have four disks of sizes 150 GB, 150
GB, 200 GB and 250 GB respectively in one RAID 5 array, then the maximum capacity is 450
GB (3 * 150 GB, the smallest disk size) and the remaining space (300 GB) is unused.
Typical applications for RAID 10 are transaction processing, relational database applications,
enterprise resource planning and other business systems. For write-intensive applications,
RAID 1 or RAID 1+0 are probably better choices, as the performance of RAID 5 will begin to
substantially decrease in a write-heavy environment.

6.9.7 Hot-spare

A RAID 1 or RAID 5 array with a hot-spare operates as a three-disk RAID 1 or RAID 5 array
with the fourth disk on standby. The standby disk automatically comes into play if a disk in the
array fails. The advantage of a hot-spare is that if a disk fails, then the array resynchronizes
automatically with the standby disk and operates at healthy array speed after the
resynchronization.
You need four hard disks installed to use RAID 10, RAID 5 or RAID 5 with hot-
spare.
154
A2 A3
B2 BP
CP C2
D1 D2
DISK 2 DISK 3
AP
B3
C3
D3
DISK 4
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