When a RAID array experiences the failure of one or more disks, it can enter degraded mode,[1] a fallback mode that generally allows the continued usage of the array, but either loses the performance boosts of the RAID technique (such as a RAID-1 mirror across two disks when one of them fails; performance will fall back to that of a normal, single drive) or experiences severe performance penalties due to the necessity to reconstruct the damaged data from error correction data.
Depending on the severity of the problem, the array may be placed into a read-only mode, either automatically or by the system administrator, until it can be corrected.
A typical case where a RAID enters degraded mode is a simple two-drive mirror after a power failure – it is unlikely the drives are in sync.
Every time blocks are written to the storage elements (physical drives, in this case), certain accounting information is updated after the write.
The RAID controller will notice that the storage elements are not in sync, will place the array in degraded mode, and – generally – will start a background resync (rebuild) operation.