Raid 0 Suxxx

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Originally posted by muchspl2@11 August 2004 - 05:24
herd good things about raid 5
my next system will be it
form what I understand if one drive fails you can fix it & even if 2 drives fail you can fix it

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Nope, only one drive. Assuming you have 3 drives, it works on the principle A(xor)B=C, where A and B are actual data on two of the drives and C is the checksum. So if one drive fails you either have A and B, A and C (B=C(xor)A) or B and C (A=C(xor)B ).The data on the three drives is laid out as follows:
ABC
CAB
BCA
ABC
etc, so that when a drive fails it only has to do the calculation to recover the lost data once in every three reads on average. Obviously there is a slight performance hit, but not too bad.

With some systems you can even replace the drive without shutting the system down (hot swapping), but even if that's not possible you only have to be down for the few minutes it takes to physically replace the drive, and can then build the raid structure while the system is running.

I was under the impression with 5 drives (in raid 5) you could replace up to 2 if need be...

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Originally posted by muchspl2@11 August 2004 - 17:18
I was under the impression with 5 drives (in raid 5) you could replace up to 2 if need be...

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That's raid 6.

Expensive to implement (you lose 2 drives), needs a very complex controller, and write performance is very poor.

Very high availability though, with 6 or more drives you can have up to 3 failed drives if the controller has been set up correctly (I believe with enough drives you could even have 4 simultaneous failures).

Chances of 2 simultaneous failures are so small that no-one has built a commercial raid 6 controller.

Therre are raid types where 2 non-adjacent drives can fail without compromising the system, usually raid 10 (with 4 drives) and raid 1E (with 5 drives), but they aren't commonly used.

OK then its raid 6
cause I knew you could replace 2 or 3 if need be, but you need a MB that supports it, I'm guessing most don't