Some info about RAIDs:
RAID stands for Redundant Array of Inexpensive Disks, the most commonly used RAIDs are:
- RAID 0 uses smaller disks to build larger volumes, for example you can use three 10 gb disks to create a 30 gb volume. There are two ways to create a volume, by concatination or stripe. In a concated volume you first use the first disk, then when its full you use the next disk and so on, on a striped volume on the other hand you write the first block on the first disk, the second block on the second disk and the third block on the third disk etc. The advantage with a striped volume is that it gives you a rather good I/O performance, the good thing about a concated volume is that you can increase the size of the volume by adding more disks, without remaking the filesystem.
- RAID 1 is commonly refered to as "mirroring", you write your data on one disk, and then mirror it to a second this, this will save your day in case of disk failures.
- RAID 3 is not as common as the other, it works like a striped RAID 0, but it also has one more disk where the system writes down the parity of the other blocks, this will save you from the failure of one disk.
- RAID 5 works like RAID 3 but the parity is spread out over the disks instead of being collected on a single disk. This will also save you from one disk failure.
- RAID 0+1 is a striped/concated volume that is mirrored, this will save you in case of disk failures on one of the mirrors.
- RAID 1+0 means that you mirror disks, then stripes or concats the mirrored disks, this will save you from a number of disk failures.
| Raid Type: | Cost: | Reability: | I/O performance: |
|---|---|---|---|
| Raid 0 Concat |
No extra | Worse | Read - Better Write - Worse |
| Raid 0 Stripe |
No extra | Worse | Read - Better Write - Better |
| Raid 1 (mirror) |
Double | Better | Read - Better Write - Worse |
| Raid 0+1 | Double | Better | Read - Better Write - Better(if striped) |
| Raid 1+0 | Double | Best | Read - Better Write - Better(if striped) |
| Raid 5* | Little more | Fair | Read - Better Write - Poor |
Some comments about RAID 5:
These figures are for SW RAID, when using HW raid the performance is a lot
better than SW RAID. Just dont forget to add some spare disks.
The Write performance for a SW RAID 5 is bad, but if one of the disk in the
RAID 5 fails (maximum one disk can fail in a RAID 5) the read and write
performance will be even worse...
