ZyXEL Communications NSA-2401 用户手册

 Chapter 6 Storage Screens

NSA-2401 User’s Guide

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. Re-synchronize 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

.

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.

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. 

Table 41   RAID 5