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Raid Overhead

post #1 of 6
Thread Starter 
I have been searching through many websites but no one gives hard numbers.

I need the above for RAIDs 3-6.

Raid 0 for example has an overhead of 0% and a Fault tolerance of zero.
Raid 1 for example has an overhead of 50% and a fault tolderance of N -1.

I was starting to think though that the fault tolerance is simply the number of drives you can lose without losing data. So Raid 1 with a minimum of two disks has a fault tolerance of 1, 2-1???

EDIT: Figured out the fault tolerance, just need to find hard numbers for overhead.
Edited by joemaniaci - 4/25/11 at 1:41pm
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post #2 of 6
This might not exactly answer your question, but it's important to keep in mind that RAID gives you redundency. Because of this a simple virus on any of the disks will be mirrored on the others, which could, depending on the virus destroy your data on all your drives.

In that scenario you have 0% fault tolerence with regards to your data, but your disks would of course still be operational.

Of course if you're running a NAS or Drobo then this shouldn't be a problem. Otherwise i'd say i agree with your analogy with regards to fault tolerence.
post #3 of 6
Check out this site;

http://raid.com/04_00.html

Good animations to show the details and they list some of the overheads...
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post #4 of 6
So overhead is %'age of total drive space you get to use for data? I associated overhead with performance loss from the controller until now.

I *think* an N disk raid3,4 and 5 would be ((N-1)/N)*100 (assuming all the drives were the same size), and an N disk raid6 would be ((N-2)/N)*100 (again with drives the same size).
Edited by Darren9 - 4/25/11 at 2:20pm
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post #5 of 6
Thread Starter 
Quote:
Originally Posted by Darren9 View Post
So overhead is %'age of total drive space you get to use for data? I associated overhead with performance loss from the controller until now.

I *think* an N disk raid3,4 and 5 would be ((N-1)/N)*100 (assuming all the drives were the same size), and an N disk raid6 would be ((N-2)/N)*100 (again with drives the same size).
yeh, it turns out that 100/N where N is the number of disks pretty much works for most of them.

The overhead comes, for most of them, the number of I/O operations required before actually writing to the RAID. There was another way of looking at overhead while searching through the several dozen websites that I did, but I can't remember what it is called now.
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post #6 of 6
Also keep in mind that the overhead is also dependent upon the controller. High end raid cards offer all kinds of processing and cache operations not found in cheaper cards. A $1500 Adaptec card will ALWAYS offer better performance than an on board controller (EDIT: Except perhaps in raid 0).

Some of the operating overhead is also mitigated (mostly) by the high end cards due to the above mentioned features.
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