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Software RAID on Linux is a snap, thanks to the mdadm tool
that comes with most modern Linux distributions. Software RAID, particularly
RAID1, is an inexpensive way to create instant backups and protect your systems
against data loss in the event that one drive fails.

To create a RAID device, execute the following command:

# mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 \
/dev/hda1 /dev/hdc1

There are two ways to obtain information about the RAID
array. You can query the device with mdadm, which provides detailed information
on a particular device, or you can get an overview of the entire RAID system by
looking at /proc/mdstat directly. For example:

# cat /proc/mdstat
Personalities : [raid1]
read_ahead 1024 sectors
md0 : active raid1 hda1[0] hdc1[1]
56261376 blocks [2/2] [UU]

Here you can see that the personality is RAID1, the device
/dev/md0 is active, and both disks are active (noted by [UU]). To obtain more detailed
information about this device, use the following command:

# mdadm --detail /dev/md0

This will print out a variety of information about the
device, including when it was created, its size, and the time it was last

If you want to change drives, you can remove a device from
the array. This command will prevent the partition /dev/hdc1 from appearing in
the /dev/md0 array:

# mdadm /dev/md0 -r /dev/hdc1

You can also add the device back into the array. Here’s how:

# mdadm /dev/md0 -a /dev/hdc1

While RAID management can be extremely complex, simple drive
mirroring (like this RAID1 setup) is fairly easy. In addition, it offers a more
flexible and cost-effective backup solution than expensive tape or removable
media solutions.