ZyXEL Communications NSA210 - Manual (Page 154)

Chapter 7 Storage
NSA210 User’s Guide
154
7.5.2 Storage Methods
This section contains theoretical background on JBOD, PC Compatible Volume and
the RAID levels used on the NSA. Redundant Array of Independent Disks (RAID) is
a method of storing data on multiple disks to provide a combination of greater
capacity, reliability, and/or speed.
These are some terms that you need to know in order to understand storage
systems.
• Mirroring
In a RAID system using mirroring, all data in the system is written
simultaneously to two hard disks instead of one. This provides 100% data
redundancy as if one disk fails the other has the duplicated data. Mirroring
setups always require an even number of drives.
• Duplexing
Like in mirroring, all data is duplicated onto two distinct physical hard drives but
in addition it also duplicates the hardware that controls the two hard drives (one
of the drives would be connected to one adapter and the other to a second
adapter).
RAID 1
RAID 1 creates an exact copy (or mirror) of a set of data on another disk. This is
useful when data backup is more important than data capacity. The following
figure shows two disks in a single RAID 1 volume with mirrored data. Data is
duplicated across two disks, so if one disk fails, there is still a copy of the data.
As RAID 1 uses mirroring and duplexing, a RAID 1 volume needs an even number
of disks (two or four for the NSA).
RAID 1 capacity is limited to the size of the smallest disk in the RAID array. For
example, if you have two disks of sizes 150 GB and 200 GB respectively in one
RAID 1 volume, then the maximum capacity is 150 GB and the remaining space
(50 GB) is unused.
Table 23 RAID 1
A1 A1
A2 A2
A3 A3
A4 A4
DISK 1 DISK 2