Managing RAID on Linux

Examining Arrays Using /proc/mdstat

The best way to find out if there are already software arrays connected to your system is to use the /proc filesystem. /proc is an interface to important kernel data structures. You can get valuable information about the state of a running Linux system by looking at special files in /proc. These special files act as a window into the running kernel. The /proc filesystem is essential for monitoring the health of any software array.

Once the RAID patches are applied to a kernel and the system is booted using the new kernel, the pseudofile /proc/mdstat will provide information about the current state of RAID devices and the md driver. Once arrays are configured and activated, information about their status will also be displayed. Initially, /proc/mdstat merely reports the available RAID levels:

$ cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid5]
read_ahead not set
unused devices: <none>

This /proc/mdstat indicates that support for linear mode RAID devices, as well as RAID-0, RAID-1, and RAID-4/5, is compiled into the running kernel. There are currently no active arrays. When there are active arrays, information about each one is reported by /proc/mdstat. In this example, there is an active RAID-1 with two member disks (/dev/sdb1 and /dev/sdc1) and one spare disk (/dev/sdd1):

# cat /proc/mdstat Personalities : [linear] [raid0] [raid1] [raid5] read_ahead 1024 sectors md0 : active raid1 sdd1[2] sdc1[1] sdb1[0] 17920384 blocks [2/2] [UU] ...