dinsdag, maart 15, 2005

Raid 0+1 is not 1+0

For years I’ve been trying to explain the difference between these two, only to find a lot of people thinking it is the same thing. Here is the difference:

Raid 0 + 1In a Raid 0+1 system, first a Raid 0 is applied on the disks. The result of this operation is speed. Next, on top of the Raid 0 (usualy done by software) a second Raid is applied, which is a Raid 1. The result of this second Raid is for security. When you look at this configuration, you might think everything is nice and dandy, but this is actually the worst kind of config, and is never put on hardware Raid controllers. This is very bad. First : Security – When one disk fails, the system has 2 disks down (both halves of a Raid 0) this means that only the remaining Raid 0 is still on-line. If a disk fails in that Raid 0 you are in SH.T. This means that only the second disk of the failed Raid 0 can fail (1 chance out of 3) and your system is still working. Second :  Speed. Again, if the system goes bad, and a disk fails, you are working at half the speed. And when you restore, the system has to copy the RaidSet from one Raid 0 onto the other Raid 0 (all disks have to work).

A much cleaner way to do this is Raid 1 + 0 :

Raid 1 + 0Two Raid 1 are created. Across those two Raid 1, a Raid 0 is created. The result may not be obvious at first, but if you fail a disk, the system still works on 3 disks(faster) if a disk fails, 2 chances out of 3, your system is still ok. When you are rebuilding, it just copies 1 disk (of the failed mirror) to the other.

This is the kind of Raidset which is usually implemented in hardware Raid controllers (if yours does not… get another).

HP (Compaq) has been advertising for years that they were using 0+1 whereas they were really using 1+0 which is far superior.

You can tell very easily if your system is using 0+1 or 1+0 .. if you remove a disk and put it back.. if only 2 disks are working to perform the restore : 1+0 .. otherwise 0+1.

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