ZyXEL Communications NAS540, User Manual

Page: 100 / 372

Chapter 8 Storage Manager
Cloud Storage User’s Guide
100
1 array 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 array needs an even number of disks (two or
four for the NAS).
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 array, then the maximum
capacity is 150 GB and the remaining space (50 GB) is unused.
Typical applications for RAID 1 are those requiring high fault tolerance without need of large
amounts of storage capacity or top performance, for example, accounting and financial data, small
database systems, and enterprise servers.
RAID 6
RAID 6 can tolerate two simultaneous drive failures by calculating dual distributed parity data on
striped data across disks. Dual parity provides extra data protection, however, it is slower to write
than most other RAID levels.
RAID 6 uses parity to store redundant data on space equal to the size of two disks for later data
recovery. Therefore, on a RAID 6 array, only 50% of the space is available as usable capacity. If you
have four disks of sizes 1TB, 1TB, 2TB, 2TB respectively in one RAID 6 array, then the maximum
capacity of the array is the capacity of the smallest drive (1TB, 1TB, 2TB, 2TB) * (Number of disks
- 2) = 1TB * (4-2) = 2TB. The remaining space (2 TB) is unused.
Table 33
RAID 1
A1 A1
A2 A2
A3 A3
A4 A4
DISK 1 DISK 2
Table 34
RAID 6
RAID 6
A1 A2 AP AQ
B1 BP BQ B2
CP CQ C1 C2
DQ D1 D2 DP
DISK 1 DISK 2 DISK 3 DISK 4