In a RAID 5 config, you need at least 3 drives (you can have more however). A small part of every drive is then reserved for recovery data for the other drives. So two drives together hold all the data needed to restore a 3rd drive.
The Raid Controllers in the Dell Poweredge Servers are hot swappable. So you can just pull the faulty disk and then insert a new replacement drive in the same slot (Needs to be the same type and size or bigger. However, if the disk is bigger than the other 2, then only the size of the two other disks will be used on the new drive). The Raid controller will then see that there is a new disk and automatically start the recovery process. You will be able to see this when you open the Controller software. Once the recovery is done, the RAID 5 will be fully functional again.
While one of the disks is faulty or missing the two remaining disks will continue to work, but at the severely degraded performance. After the rebuild the performance will be back to the way it was before.
The rebuild can take more than an hour, depending on the disk sizes used.
There is also an option on some Controllers to select one drive as a "spare". Then you insert one more disk and set it as spare. It will not start working unless one of the other disks in the Raid fails. That's when it is used automatically as a replacement.
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