Managing RAID on Linux

Managing Disk Failures

When a member disk of a RAID-1, RAID-4, or RAID-5 fails, the array enters into degraded mode. Degraded mode means that both performance and redundancy are impacted. RAID-0 and linear mode never enter into degraded mode because they do not support redundancy. If a disk in either a RAID-0 or linear mode configuration fails, the array stops. Unless the disk can be repaired, data will be lost.

RAID-1 can withstand at least a single disk failure. For a RAID-1 of n member disks, n-1 disks can fail before service is interrupted. When all disks in a RAID-1 fail, the array is no longer functional. In addition, parallel read performance of RAID-1 is affected by disk failures. For example, a RAID-1 consisting of three disks can potentially achieve parallel reads of up to three times the throughput of a single member disk. If a single disk fails, parallel read performance is reduced by a factor of one. An interesting side effect of disk failures under RAID-1 is that write performance will actually improve during degraded operation. That’s because the number of writes that occurs is multiplied by the number of member disks in the array. As a RAID-1 loses member disks, the number of writes per I/O operation decreases.

RAID-4 and RAID-5 deal with disk failures in the same way. They can each survive only a single disk failure. Disk failures in RAID-4 and RAID-5 considerably impact array performance. Each time data is read from the array, the system must perform parity reconstruction ...