For many desktop configurations, RAID level 1 (RAID 1) is appropriate because it can be set up with only two drives. For servers, RAID 5 or 6 is commonly used.
Although Table 6-3 specifies the number of drives required by each RAID level, the Linux RAID system is usually used with disk partitions, so a partition from each of several disks can form one RAID array, and another set of partitions from those same drives can form another RAID array.
RAID arrays should ideally be set up during installation, but it is possible to create them after the fact. The mdadm command is used for all RAID administration operations; no graphical RAID administration tools are included in Fedora.
6.2.1.1. Displaying Information About the Current RAID Configuration
The fastest way to see the current RAID configuration and status is to display the contents of /proc/ mdstat :
$ cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 hdc1[1] hda1[0]
102144 blocks [2/2] [UU]
md1 : active raid1 hdc2[1] hda3[0]
1048576 blocks [2/2] [UU]
md2 : active raid1 hdc3[1]
77023232 blocks [2/1] [_U]
This display indicates that only the raid1 ( mirroring) personality is active, managing three device nodes:
md0
This is a two-partition mirror, incorporating /dev/hda1 (device 0) and /dev/hdc1 (device 1). The total size is 102,144 blocks (about 100 MB). Both devices are active.
md1
This is another two-partition mirror, incorporating /dev/hda3 as device 0 and /dev/hdc2 as device 1. It's 1,048,576 blocks long (1 GB), and both devices are active.
md2
This is yet another two-partition mirror, but only one partition ( /dev/hdc3 ) is present. The size is about 75 GB.
The designations md0 , md1 , and md2 refer to multidevice nodes that can be accessed as /dev/md0 , /dev/md1 , and /dev/md2 .
You can get more detailed information about RAID devices using the mdadm command with the -D (detail) option. Let's look at md0 and md2 :
# mdadm -D /dev/md0
/dev/md0:
Version : 00.90.03
Creation Time : Mon Aug 9 02:16:43 2004
Raid Level : raid1
Array Size : 102144 (99.75 MiB 104.60 MB)
Device Size : 102144 (99.75 MiB 104.60 MB)
Raid Devices : 2
Total Devices : 2
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Tue Mar 28 04:04:22 2006
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
UUID : dd2aabd5:fb2ab384:cba9912c:df0b0f4b
Events : 0.3275
Number Major Minor RaidDevice State
0 3 1 0 active sync /dev/hda1
1 22 1 1 active sync /dev/hdc1
# mdadm -D /dev/md2
/dev/md2:
Version : 00.90.03
Creation Time : Mon Aug 9 02:16:19 2004
Raid Level : raid1
Array Size : 77023232 (73.46 GiB 78.87 GB)
Device Size : 77023232 (73.46 GiB 78.87 GB)
Raid Devices : 2
Total Devices : 1
Preferred Minor : 2
Persistence : Superblock is persistent
Update Time : Tue Mar 28 15:36:04 2006
State : clean, degraded
Active Devices : 1
Working Devices : 1
Failed Devices : 0
Spare Devices : 0
UUID : 31c6dbdc:414eee2d:50c4c773:2edc66f6
Events : 0.19023894
Number Major Minor RaidDevice State
0 0 0 - removed
1 22 3 1 active sync /dev/hdc3
Note that md2 is marked as degraded because one of the devices is missing.
6.2.1.2. Creating a RAID array
To create a RAID array, you will need two block devicesusually, two partitions on different disk drives.
If you want to experiment with RAID, you can use two USB flash drives; in these next examples, I'm using some 64 MB flash drives that I have lying around. If your USB drives are auto-mounted when you insert them, unmount them before using them for RAID, either by right-clicking on them on the desktop and selecting Unmount Volume or by using the umount command.
The mdadm option --create is used to create a RAID array:
# mdadm --create -n 2 -l raid1 /dev/md0 /dev/sdb1 /dev/sdc1
mdadm: array /dev/md0 started.
There are a lot of arguments used here:
--create
Tells mdadm to create a new disk array.
-n 2
The number of block devices in the array.
-l raid1
The RAID level.
/dev/md0
The name of the md device.
/dev/sdb1 /dev/sdc1
The two devices to use for this array.
/proc/mdstat shows the configuration of /dev/md0 :
# cat /proc/mdstat
Personalities : [raid1]