Chris Tyler - Fedora Linux

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.

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]

md0 : active raid1 sdc1[1] sdb1[0]

63872 blocks [2/2] [UU]

unused devices:

If you have three or more devices, you can use RAID 5, and if you have four or more, you can use RAID 6. This example creates a RAID 5 array:

# mdadm --create -n 3 -l raid5 /dev/md0 /dev/sdb1 /dev/sdc1 /dev/sdf1

mdadm: largest drive (/dev/sdb1) exceed size (62464K) by more than 1%

Continue creating array? y

mdadm: array /dev/md0 started.

Note that RAID expects all of the devices to be the same size. If they are not, the array will use only the amount of storage equal to the smallest partition on each of the devices; for example, if given partitions that are 50 GB, 47.5 GB, and 52 GB in size, the RAID system will use 47.5 GB in each of the three partitions, wasting 5 GB of disk space. If the variation between devices is more than 1 percent, as in this case, mdadm will prompt you to confirm that you're aware of the difference (and therefore the wasted storage space).

Once the RAID array has been created, make a filesystem on it, as you would with any other block device:

# mkfs -t ext3 /dev/md0

mke2fs 1.38 (30-Jun-2005)

Filesystem label=

OS type: Linux

Block size=1024 (log=0)

Fragment size=1024 (log=0)

16000 inodes, 63872 blocks

3193 blocks (5.00%) reserved for the super user

First data block=1

Maximum filesystem blocks=65536000

8 block groups

8192 blocks per group, 8192 fragments per group

2000 inodes per group

Superblock backups stored on blocks: 8193, 24577, 40961, 57345

Writing inode tables: done

Creating journal (4096 blocks): done

Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 28 mounts or 180 days, whichever comes first. Use tune2fs -c or -i to override.

Then mount it and use it:

# mkdir /mnt/raid

# mount /dev/md0 /mnt/raid

Alternately, you can use it as a PV under LVM. In this example, a new VG test is created, containing the LV mysql :

# pvcreate /dev/md0

Physical volume "/dev/md0" successfully created

# vgcreate test /dev/md0

Volume group "test" successfully created

# lvcreate test --name mysql --size 60M

Logical volume "mysql" created

# mkfs -t ext3 /dev/test/mysql

mke2fs 1.38 (30-Jun-2005)

...(Lines skipped)...

This filesystem will be automatically checked every 36 mounts or 180 days, whichever comes first. Use tune2fs -c or -i to override.