Notes on mdadm.
Useful commands
Get basic info on RAID status:
cat /proc/mdstat
Read config:
cat /etc/mdadm.conf
Get detailed info, e.g. for /dev/md1 array:
mdadm --detail /dev/md1
Assemble an array
To assemble /dev/md1 array from disk partitions /dev/sda1 and /dev/sdb1:
mdadm --assemble /dev/md1 /dev/sda1 /dev/sdb1
Write config to /etc/mdadm.conf
When you’ve assembled your arrays, you can add the config to /etc/mdadm.conf so that you can easily start the arrays in the future.
mdadm --detail --scan >> /etc/mdadm.conf
The arrays can now be assembled with the following command:
mdadm --assemble --scan
Adding a hot spare to an existing RAID1 array
Say we have /dev/sda and /dev/sdb in an array /dev/md1 and we want to add a blank disk as a hot spare.
Copy the partition table from /dev/sda to /dev/sdc:
Warning: Repartitioning a disk with sfdisk will destroy its data.
sfdisk -d /dev/sda | sfdisk /dev/sdc
Add the spare disk to the array:
mdadm --add /dev/md1 /dev/sdc1
mdadm --add /dev/md2 /dev/sdc2
(Two partitions participating in RAID1).
Check with mdadm --detail /dev/mdN where N is the array number. It should show two active devices and one spare.
E.g. In this example /dev/sdc1 and /dev/sdc2 are already partitioned:
me@pc ~ $ sudo mdadm --add /dev/md1 /dev/sdc1
Password:
mdadm: added /dev/sdc1
me@pc ~ $ sudo mdadm --add /dev/md2 /dev/sdc2
mdadm: added /dev/sdc2
me@pc ~ $ cat /proc/mdstat
Personalities : [raid1]
md1 : active raid1 sdc1[2](S) sda1[3] sdb1[1]
190272 blocks super 1.2 [2/2] [UU]
md2 : active raid1 sdc2[2](S) sda2[0] sdb2[1]
488191168 blocks [2/2] [UU]
unused devices: <none>
