Data is distributed across the drives in one of several ways, referred to as RAID levels, depending on the required level of redundancy and performance. The different schemas, or data distribution layouts, are named by the word RAID followed by a number, for example RAID 0 or RAID 1. Each schema, or RAID level, provides a different balance among the key goals:
reliability, availability, performance, and capacity.
RAID levels greater than RAID 0 provide protection against unrecoverable sector read errors, as well as against failures of whole physical drives.
Level |
Striping |
Mirroring |
Parity |
Disk Failure |
Minimum |
Details |
X |
|
|
0 |
2 |
|
|
|
X |
|
1 |
2 |
|
|
X |
|
X |
1 |
3 |
|
|
X |
|
X |
2 |
4 |
RAID 5 + Additional parity block |
|
X |
X |
|
1 per mirror set |
4 |
RAID 0 + RAID 1 |
|
RAID 50 |
X |
|
X |
|
6 |
RAID 0 + RAID 5 |
RAID 60 |
X |
|
X |
|
8 |
RAID 0 + RAID 6 |
Allows you to write data across multiple physical disks instead of just one physical disk. RAID 0 involves partitioning each physical disk storage space into 64 KB stripes. These stripes are interleaved in a repeated sequential manner. The part of the stripe on a single physical disk is called a stripe element.
For example, in a four-disk system using only RAID 0, segment 1 is written to disk 1, segment 2 is written to disk 2, and so on. RAID 0 enhances performance because multiple physical disks are accessed simultaneously, but it does not provide data redundancy (Figure 1 (English only)).
Figure 1: RAID 0
With RAID 1, data written to one disk is simultaneously written to another disk. If one disk fails, the contents of the other disk can be used to run the system and rebuild the failed physical disk.
The primary advantage of RAID 1 is that it provides 100 percent data redundancy. Because the contents of the disk are completely written to a second disk, the system can sustain the failure of one disk. Both disks contain the same data at all times. Either physical disk can act as the operational physical disk (Figure 2 (English only)).
Figure 2: RAID 1
Figure 3: RAID 5
Figure 4: RAID 6
Figure 5: RAID 10