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What are the general replacement requirements for a failed RAID 1 (mirrored) or 5 (parity) hard drive?

Does the replacement drive have to be exactly the same (1) model and/or (2) size of the original failed drive? With regard to replacement size, I assume it would have to be at least the same size or larger.

Can I use a solid-state drive in place of a traditional magnetic drive? Or vice-versa?

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In theory, you can mix-and-match the disk types as much as you want as long as the replacement disks are all of the same greater size.

In practice, you really do NOT want to mix the drive types: it introduce a level of complexity (because the disk physical layout are different) that usually causes the whole thing to be much slower afterward. Depending on the exact controller type, it might also simply not work.

Also, the performance of the whole array will be linked to the performance of the slowest disk: using an SSD in there will not help at all.

The only occasions when I could imagine this to be of any advantage would be:

  • Hot-replace the disks with a faster model (yank a disk out, replace, wait for the rebuild to finish, proceed with the next one).
  • You have a failure and have no exact replacement will be available in a reasonable time frame. In such a case using a replacement disk of different capacity could make your whole operation more secure until you get that proper replacement.

It's a really good time to check your backups, though: a single failed read on any of the disks in the array during rebuild and your data is toast. That's why I wouldn't recommend rebuilding the arrays any more frequently than you absolutely have to.

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Thanks for the thorough answer, Stephane! =) –  Jeff Nov 23 '11 at 12:33
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1: Yes. 2: Yes, and with the remaining space you could create a simple partition and use it, but you should put in it the less frequent data that you should access to, because each access to it would decrease the performence of the RAID setup.

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Its recommended to find the same type of disk. This limits the complexities. The RAID controller might not like some oddball disk in there. If you cannot source this disk, pick the most similiar disk possible. If you have a Seagate ES400 60gig and you can't find it, then buy the 80 or 100gb model. In a pinch, most disks will work, but why take that chance unless you absolutely have to?

SSD's wont do you any good. First off, the array is only as fast as its slowest disk. Secondly, SSD's are a reliability nightmare and your RAID controller may just reject it. I could see the argument to build a fresh array using a SLC (not MLC!!) SSD with a RAID card known to work well with SSD and which, ideally, can do TRIM on the array.

I'd also google your RAID card model + name of proposed drive and see what comes up.

Lastly, you need an enterprise grade drive to do RAID. Don't just buy whats on sale at newegg. Consumer drives aren't tested or designed to do RAID and can cause issues (going to sleep, overheat, greatly slow down the array, corrupt data).

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