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# RAID 5 question

Posted on 2006-05-19
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Is the amount of drives that could go bad, and the RAID still remain functional, proportionate to the amount of drives in the array?

Example - if I have a 3 drive array, and one goes bad, it's still up and running until another goes bad, then all is lost.

If I have an 8 drive array, is it still only 1 drive that could go out, or could more and still remain functional?
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Question by:Purple_Tidder
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Callandor earned 1000 total points
ID: 16720183
Correct - for RAID-5, it's one drive that you can have fail and still be functional ("degraded state").
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Assisted Solution

Lee W, MVP earned 1000 total points
ID: 16720186
No, With RAID 5, regardless of how many disks in the array, if more than one fails, you lose everything
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Author Comment

ID: 16720188
Whew, fast fast guys.  Thanks a bunch.
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ID: 16720194
Split, you guys are same time shooters...
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ID: 16720209
Well, I guess on a related note, you guys know of any RAID type off the top of your head that will allow more than 1 to go?  Like double parity type of deal?
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Expert Comment

ID: 16720211
RAID 0 - NO disks can fail without losing everything
RAID 1 - ONE disk may fail without losing everything
RAID 5 - ONE disk may fail, regardless of the number of disks in the array, without losing everything
RAID 10 - ONE OR MORE disks may fail without losing everything.  The exact number depends on the number of drives in the RAID 0 AND WHICH drives fail.  With a RAID 10 that uses 2-three disk sets of RAID 0, you could lose all three on one set, but if you lose one on each set, you lose everything.
RAID 51 - (What I call it) uses two RAID 5 sets and mirrors them.  So a 10 disk RAID 51 can lose 2-6 disks and still be operational, depending on which ones fail.
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ID: 16720223
Wow, RAID 51, never even heard of it.  Thanks leew, may be the way to go.
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Expert Comment

ID: 16720226
You can also use a hotspare which KINDA increases the number of disks that can fail.  A hot spare will take over for a failed disk, but needs time to have the data rebuilt onto it.  So if two disks go at the same time OR before the RAID can be rebuilt onto the hot spare, then you lose everything.  But if one fails friday night and another fails sunday AND you have a hot spare, then two can fail without losing data (with some exceptions for very large arrays).
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ID: 16720240
Neat.  I'll check into that as well.
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Expert Comment

ID: 16720244
Here's some background on RAID types: http://www.acnc.com/04_01_01.html
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Expert Comment

ID: 16720247
Like I said, it's what I call it - I've not seen anyone else call it that.  But I did have to use it once on a production system which kept having disk failures.  It's the least efficient method of RAID as you only get n/2-1 drives worth of space (where n is the number of total drives used AND is an even number.

To be clear, if you have a box with 14 drives, you'll only get total usable disk space of 6 drives with "RAID 51"
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ID: 16720271
Yeah, that seems to be the way to go, they're not worried as much about storage space and cost as they are about having it always available, no matter what.  It's maybe 20 gigs. :D
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