6.3. Managing Software RAID

This section discusses software RAID configuration and management after the installation, and covers the following topics:
  • Reviewing existing software RAID configuration.
  • Creating a new RAID device.
  • Replacing a faulty device in an array.
  • Adding a new device to an existing array.
  • Deactivating and removing an existing RAID device.
  • Saving the configuration.
All examples in this section use the software RAID configuration from the previous section.

6.3.1. Reviewing RAID Configuration

When a software RAID is in use, basic information about all presently active RAID devices are stored in the /proc/mdstat special file. To list these devices, display the content of this file by typing the following at a shell prompt:
cat /proc/mdstat
To determine whether a certain device is a RAID device or a component device, run the command in the following form as root:
mdadm --query device
In order to examine a RAID device in more detail, use the following command:
mdadm --detail raid_device
Similarly, to examine a component device, type:
mdadm --examine component_device
While the mdadm --detail command displays information about a RAID device, mdadm --examine only relays information about a RAID device as it relates to a given component device. This distinction is particularly important when working with a RAID device that itself is a component of another RAID device.
The mdadm --query command, as well as both mdadm --detail and mdadm --examine commands allow you to specify multiple devices at once.

Example 6.1. Reviewing RAID configuration

Assume the system uses configuration from Figure 6.7, “Sample RAID Configuration”. You can verify that /dev/md0 is a RAID device by typing the following at a shell prompt:
~]# mdadm --query /dev/md0
/dev/md0: 125.38MiB raid1 2 devices, 0 spares. Use mdadm --detail for more detail.
/dev/md0: No md super block found, not an md component.
As you can see, the above command produces only a brief overview of the RAID device and its configuration. To display more detailed information, use the following command instead:
~]# mdadm --detail /dev/md0
        Version : 0.90
  Creation Time : Tue Jun 28 16:05:49 2011
     Raid Level : raid1
     Array Size : 128384 (125.40 MiB 131.47 MB)
  Used Dev Size : 128384 (125.40 MiB 131.47 MB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Thu Jun 30 17:06:34 2011
          State : clean
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           UUID : 49c5ac74:c2b79501:5c28cb9c:16a6dd9f
         Events : 0.6

    Number   Major   Minor   RaidDevice State
       0       3        1        0      active sync   /dev/hda1
       1       3       65        1      active sync   /dev/hdb1
Finally, to list all presently active RAID devices, type:
~]$ cat /proc/mdstat
Personalities : [raid0] [raid1]
md0 : active raid1 hdb1[1] hda1[0]
      128384 blocks [2/2] [UU]
md1 : active raid0 hdb2[1] hda2[0]
      1573888 blocks 256k chunks

md2 : active raid0 hdb3[1] hda3[0]
      19132928 blocks 256k chunks

unused devices: <none>