OK, while most people think of RAID as just for disks its really mean to describe anything. But lets talk about disks for a minute.
Different RAID levels refer to different styles of building an array. The numbers kind of imply one building on another (i.e. Raid 5 is better than Raid 4), but don't think of them as perfectly building on another.
| Raid 0 Striping | This mearly means taking two or more drives and combining them into one larger drive. There are two reasons for this, one being lazyness and one being performance. When I say lazyness I mean unless you had a single file larger than one drive the only reason for combining two drives into a single drive is so that you dont need to remember which drive your files are on. Striping can improve Performance because it doesn't just add the new disk to the end of the old disk. Every other block is written to the other device so when you are reading a file 50% of the file comes from drive 1 and 50% of the file comes from drive 2. Since drives are fairly slow devices this has a very positive impact |
Raid 1 | This refers to using two drives to store the same data. The purpose of this is to protect the data rather than improve performance. If one of the drives in a Mirror stops functioning then the data is still availible because the other member of the mirror is still availible. The disadvantage to this configuration is that you need 2x storage to save files. i.e. you want 500gb you need to supply 1tb of storage (as two seperate 500gb drives) Performance is a mixed bag. Every time you write, the machine needs to write the data twice. Every time you do a read, the machine can read 50% of the data from drive 1 and 50% of the data from drive 2. Sometimes Raid 1 is configured to do a read/verify meaning read the same sector from both drives and verify the mirror is valid. |
| Raid 2 | Cannot find anything |
| Raid 3/4 Striped with Parity I have found different explanations for 3 vs 4 for this i'm not going to discuss the differences | Simlar to a Raid 0 stripe except there is 1 extra drive that is providing a level of protection. Parity is a error correction method that works as follows X + 1 = 0, so what is the value of X Since the parity drive needs to be written to every time any of the other drives change it gets accessed far more often than the others. There is a danger that the parity drive will fail long before any of the members fail. |
| Raid 5 Parity with Striping | Raid 5 is the same idea as Raid 3/4 with the creation of parity, but the parity data is striped accross the disks as well. In this way one drive is not dedicated to the parity, and the load is shared accross all drives. Raid 5 does not mean that 5 drives must be used. Raid 5 can be done with three or more drives. People typically don't do very large (# of disk's) because of the danger of 2 drive loss |
| Raid 6 Parity with Dual Striping | Raid 5 suffers from a danger that when a drive fails, until its replaced and rebuild the array is in danger. If a second drive were to fail all of the data would be corrupt Raid 6 addresses this by having 2 parity areas accross the array, which mean you need a minimum of 4 drives per array, but the practical minimum is 6 otherwise mirroring makes more sense. Two drives could fail without loss of data. |
Raid 0 + 1 | This is a combination of Striping and Mirroring (done the wrong way). Create 2 stripe sets, then mirror them. The problem is as follows: If A and D or B and C fail, the entire array is dead |
Raid 1 + 0 (10) | More stable configuration, Create mirrored pairs, then stripe them Mirror 1 - Disks A and B |
Raid 5 + 0 (50) | Takes two Raid 5 arrays and creates a single larger drive. Raid 6 can provide same results with better protection Array 1 - Raid 5 of Disks A, B, C, D and E |
Raid 5 + 1 (51) | Can you say paranoid ? Array 1 - Raid 5 of Disks A, B, C, D and E |
http://en.wikipedia.org/wiki/RAID
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