Mdadm
mdadm is a Linux utility to manage software RAID.
Quick Usage
Creating a Raid 1 Mirror
# mdadm --zero-superblock /dev/sdb
# mdadm --zero-superblock /dev/sdc
Create the array:
# mdadm --create /dev/md0 --chunk=4 --level=1 --raid-devices=2 /dev/sdb /dev/sdc
That's it! Create the filesystem and you're done. Using 4k chunk size.
To save your settings, run:
# echo "DEVICES /dev/sdb /dev/sdc" > /etc/mdadm.conf
# mdadm --detail --scan >> /etc/mdadm.conf
# cat /etc/mdadm.conf
DEVICES /dev/sdb /dev/sdc
ARRAY /dev/md/0 metadata=1.2 name=data:0 UUID=5f486d36:37899fcb:99ad8485:088bc180
Creating a Raid 5
Create the array:
# mdadm --create /dev/md0 --chunk=128 --level=5 --raid-devices=4 /dev/sdb /dev/sdc /dev/sdd /dev/sde
You can specify hot spares with --spare-devices=N /dev/sdn
Disassemble / stop an array
# mdadm --stop /dev/md0
Reassemble an array
# mdadm --assemble --scan
You can assemble an array with specific devices using:
# mdadm --assemble /dev/md0 /dev/sdb /dev/sdc
Recovering an Array
If you want to recover data from an array of a failed system, image the hard drives in case something goes terribly wrong. Then, attempt to reassemble the array with the -o or --readonly option for read only. This will prevent mdadm from attempting to resync, recover, or reshape the array.
# mdadm --readonly --verbose --assemble /dev/md0 /dev/sdb /dev/sdc
# or
# mdadm --readonly --verbose --assemble --scan
Check on the array status:
# cat /proc/mdstat
Issues
If you're unable to reassemble an array with this error:
# mdadm --assemble /dev/md0 /dev/sdb /dev/sdc
mdadm: cannot open device /dev/sdb: Device or resource busy
mdadm: /dev/sdb has no superblock - assembly aborted
You probably forgot to define your /etc/mdadm.conf settings before rebooting and mdadm probably tried to do something with one of your disks to create /dev/md127 (hence the reason the device is busy). To free /dev/sdb in the above example, just stop /dev/md127 by running:
# mdadm --stop /dev/md127
And then retry reassembling the array again.