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Let's assume the following scenario:

  1. A RAID5 array consisting of 3 disks is happy and running
  2. One of the disks crashes
  3. We replace the broken disk with a spare one
  4. The system is up and running again

So, the question is: When exactly does the reconstruction happen? Is it right after 2., or right after 3.?

I'd bet the array doesn't do anything before we replace the disk and the RAID controller would be able to calculate data. Does something happen before we replace the disk?

EDIT: I think i'm starting to understand why some people mention that the reconstruction begins just after the disk failure. That would be the case if a spare disk (disk 4) was already connected to the raid controller. So I think nothing happens if we have only 3 disks in RAID5 until we replace the broken one.

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Reconstruction typically happens after you replace the failed disk, because you have to have a healthy disk to reconstruct to.

There are a couple things I can think of that can change this order, however.

One is the presence of a "hot spare" disk, which is a disk that's in the array, but not allocated, and designated to be used as a replacement in the event of failure. In this case, you'd see reconstruction begin following the drive failure, rather than the drive replacement.

The other factor I can think of is that some older or cheaper array controllers require manual intervention to initiate an array rebuild, in which case, you'd need to replace the disk, and then tell the array controller to start rebuilding.

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