I am running CentOS 4.4 on some old servers, and each of these servers has multiple internal disk drives. Since system availability concerns me more than the amount of storage that is available, I decided to add a hot spare to the md device that stores my data (md2). To add the hot spare, I ran the mdadm utility with the “–add” option, the md device to add the spare to, and the spare device to use:
$ /sbin/mdadm --add /dev/md2 /dev/sdh1
mdadm: added /dev/sdh1
After the spare was added, the device showed up in the /proc/mdstat output with the “(S)” string to indicate that it’s a hot spare:
$ cat /proc/mdstat
Personalities : [raid1] [raid6] [raid5] [raid4]
md1 : active raid1 sdb2[1] sda2[0]
8385856 blocks [2/2] [UU]
bitmap: 0/128 pages [0KB], 32KB chunk
md2 : active raid5 sdh1[5](S) sdg1[4] sdf1[3] sde1[2] sdd1[1] sdc1[0]
976751616 blocks level 5, 64k chunk, algorithm 2 [5/5] [UUUUU]
bitmap: 3/233 pages [12KB], 512KB chunk
md0 : active raid1 sdb1[1] sda1[0]
235793920 blocks [2/2] [UU]
bitmap: 7/225 pages [28KB], 512KB chunk
unused devices: