Managing RAID on Linux

Automatic Failover to a Spare Disk

Disk replacement can be handled automatically by the Linux kernel, or by RAID controllers, if hot-spare disks have been allocated to the array. Insertion of these disks should be fairly automatic. The md driver takes action immediately following the first read or write to an array in degraded mode and reports any changes via syslogd. When the hot-spare is introduced, reconstruction commences.

Jun 27 18:15:53 apathy kernel: md: updating md0 RAID superblock on device
Jun 27 18:15:53 apathy kernel: sde1 [events: 00000002](write) sde1's sb offset:
    17920384
Jun 27 18:15:53 apathy kernel: md: recovery thread got woken up ...
Jun 27 18:15:53 apathy kernel: md0: resyncing spare disk sde1 to replace failed disk

First, the md driver updates the RAID superblock for /dev/md0 to reflect the fact that the failed disk is no longer a member of the array and that the spare-disk is now an active member. Next, information about the new member’s event counter is reported. (The event counter is simply a report of how many RAID configuration changes have been executed on the device.) Next, reconstruction commences and the mdrecoveryd process rebuilds the array. You can also examine /proc/mdstat to monitor the reconstruction process.

When the process is completed, syslogd reports on the new status of the array:

Jun 27 18:35:33 apathy kernel: md: md0: sync done. Jun 27 18:35:33 apathy kernel: RAID5 conf printout: Jun 27 18:35:33 apathy kernel: --- rd:3 wd:2 fd:1 Jun 27 18:35:33 ...