Degraded array
A degraded array is a RAID array that has lost one or more member disks but can still serve data from the disks that remain.
Why does an array become degraded?
A disk stops responding, reports errors, or is removed, and the controller or software takes it out of the array. On Linux software RAID, a write error is enough for the driver to mark the disk faulty and continue on the remaining disks. The disk may have a physical failure.
What should you do if an array reports degraded?
If the data matters and there is no current backup, copy it off before you replace disks or start a rebuild. Some or all of the array’s safety margin has gone. A rebuild regenerates everything that was on the failed disk from the disks that remain, so it reads across all of them. If the array has already dropped a second disk, or the volume has gone, see RAID 5 data recovery.
What might you notice?
- The NAS or controller shows the pool or volume as degraded
- One disk is listed as failed, missing or offline
- The array is slower than usual
- A rebuild or repair is being offered
From the lab
Observed
A Dell PowerEdge T110 II server came in with a three-drive RAID 5 array. One drive had dropped off the array a long time ago. The engineers copied the disks sector by sector, identified the RAID parameters, and left that drive out of the reconstruction. The result was a full file tree without errors. Dell PowerEdge RAID 5 case study
From R3's lab records
Related terms
Common misconceptions
Does degraded mean data has been lost?
Not by itself. A degraded array can still be read, which is the point of the redundancy. What has been lost is the protection: with single parity, the next failure is the one that loses data.