# RAID 5 question

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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Commented:
Correct - for RAID-5, it's one drive that you can have fail and still be functional ("degraded state").
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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 Commented:
Whew, fast fast guys.  Thanks a bunch.
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Author Commented:
Split, you guys are same time shooters...
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Author Commented:
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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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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Author Commented:
Wow, RAID 51, never even heard of it.  Thanks leew, may be the way to go.
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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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Author Commented:
Neat.  I'll check into that as well.
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Commented:
Here's some background on RAID types: http://www.acnc.com/04_01_01.html
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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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Author Commented:
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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