Managing RAID on Linux

RAID-1 (Mirroring)

Setting up a mirror is slightly different from using linear mode or RAID-0. We already know that mirroring replicates data across all member disks. This allows a RAID-1 to continue functioning even if a disk fails. The simplest RAID-1 configuration must contain at least two member disks. In this example, /dev/sdb1 and /dev/sdc1 are member disks of the RAID-1 at /dev/md0:

# A RAID-1 with two member disks

raiddev                /dev/md0
 raid-level            1
 nr-raid-disks         2
 chunk-size            64

 device                /dev/sdb1
 raid-disk             0

 device                /dev/sdc1
 raid-disk             1

chunk-size has no effect on RAID-1 because no disk striping is involved. But chunk-size is still required as a placeholder. Note also that the persistent-superblock isn’t needed for RAID-1. Use mkraid to create this array:

# mkraid /dev/md0
handling MD device /dev/md0
analyzing super-block
disk 0: /dev/sdb1, 17920476kB, raid superblock at 17920384kB
disk 1: /dev/sdc1, 17920476kB, raid superblock at 17920384kB

Or, using mdadm:

# mdadm -Cv -l1 -n2 /dev/md0 /dev/sd{b,c}1
mdadm: array /dev/md0 started.

Whenever a new mirror is created, resynchronization occurs:

# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid5]
read_ahead 1024 sectors
md0 : active raid1 sdc1[1] sdb1[0]
      17920384 blocks [2/2] [UU]
      [=  =  =  =  =  =  =  =>............]  resync = 40.2% (7212864/17920384)
finish=6.4min speed=27652K/sec
unused devices: <none>

Now /proc/mdstat reports information about the array and also includes information about the resynchronization process. Resynchronization ...