Fedora Linux

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]
md0 : active raid1 sdc1[1] sdb1[0]
63872 blocks [2/2] [UU]
unused devices: <none>

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