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Does this mean setting aside one more disk as a hotspare than you actually need for RAID5?

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Yes, a hot spare is a disk that is set aside and will be immediately called upon to rebuild a RAID set should one of the drives in that RAID set die. This is opposed to a cold spare which is a hard drive that is not set to automatically be chosen as a rebuild disk. A cold spare is a fancy name for one of those hard drives that you keep in your desk drawer should something bad happen to one of the servers.

The functionality for a hot spare can be implemented in unique ways depending on the RAID controller that you're using, so check up on that. I've heard of RAID controllers not wanting to keep the array on the hot spare after a failure and will rebuild the array when you replace the failed drive. That seems counterintuitive to me since I don't want datasets being rebuilt anymore than necessary.

Have a look at ACNC's easy to follow animated RAID tutorial. When searching for this link to give you, I was slightly annoyed that they didn't address the topic of hotspares. However, it explains many of the other RAID levels... even the rarely heard of RAID 2 and 4.

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great info! thanks for the link too! & your support –  John-ZFS Apr 23 '10 at 6:40
    
For a specific example, HP smartarray 6400 controllers rebuilds the array again after the failed drive is replaced. Probably other smartarray controllers as well, as they probably all use more or less the same firmware. –  janneb Apr 23 '10 at 7:20
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Correct. And when (if... knock on wood) one of the drives in the array fails, the hot spare will be switched into the array and the array will be rebuilt, on the fly.

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The trick here is: Immediately. If it is friday night, you may not be in the office till monday. hot-spare goes in immediately. –  TomTom Apr 23 '10 at 7:43
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