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22.3. Using mdadm to Configure RAID-Based and Multipath Storage

Similar to other tools comprising the raidtools package set, the mdadm command can be used to perform all the necessary functions related to administering multiple-device sets. This section explains how mdadm can be used to:
  • Create a RAID device
  • Create a multipath device

22.3.1. Creating a RAID Device With mdadm

To create a RAID device, edit the /etc/mdadm.conf file to define appropriate DEVICE and ARRAY values:
DEVICE /dev/sd[abcd]1
ARRAY /dev/md0 devices=/dev/sda1,/dev/sdb1,/dev/sdc1,/dev/sdd1
In this example, the DEVICE line is using traditional file name globbing (refer to the glob(7) man page for more information) to define the following SCSI devices:
  • /dev/sda1
  • /dev/sdb1
  • /dev/sdc1
  • /dev/sdd1
The ARRAY line defines a RAID device (/dev/md0) that is comprised of the SCSI devices defined by the DEVICE line.
Prior to the creation or usage of any RAID devices, the /proc/mdstat file shows no active RAID devices:
Personalities :
read_ahead not set
Event: 0
unused devices: none
Next, use the above configuration and the mdadm command to create a RAID 0 array:
mdadm -C /dev/md0 --level=raid0 --raid-devices=4 /dev/sda1 /dev/sdb1 /dev/sdc1 \
Continue creating array? yes
mdadm: array /dev/md0 started.
Once created, the RAID device can be queried at any time to provide status information. The following example shows the output from the command mdadm --detail /dev/md0:
Version : 00.90.00
Creation Time : Mon Mar  1 13:49:10 2004
Raid Level : raid0
Array Size : 15621632 (14.90 GiB 15.100 GB)
Raid Devices : 4
Total Devices : 4
Preferred Minor : 0
Persistence : Superblock is persistent

Update Time : Mon Mar  1 13:49:10 2004
State : dirty, no-errors
Active Devices : 4
Working Devices : 4
Failed Devices : 0
Spare Devices : 0

Chunk Size : 64K

      Number   Major   Minor   RaidDevice State
         0       8        1        0      active sync   /dev/sda1
         1       8       17        1      active sync   /dev/sdb1
         2       8       33        2      active sync   /dev/sdc1
         3       8       49        3      active sync   /dev/sdd1
           UUID : 25c0f2a1:e882dfc0:c0fe135e:6940d932
         Events : 0.1

  Published under the terms of the GNU General Public License Design by Interspire