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.

The array keeps answering reads and writes by using its redundancy: a mirror copy, or parity. That redundancy is now being used up. A RAID 5 array that is missing one disk has none left, so a second failed or unreadable disk stops the missing data from being calculated.

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

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.

Sources

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