Let's assume the following scenario:
- A RAID5 array consisting of 3 disks is happy and running
- One of the disks crashes
- We replace the broken disk with a spare one
- 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.