Linux software RAID with mdadm

by Vincent Danen | Apr 11, 2005 7:00:00 AM

Tags: Storage, OPEN SOURCE, Vincent Danen, Linux software, RAID, RAID1, software RAID, Linux, Linux Tips Newsletter

Takeaway: Software RAID is an inexpensive way to create instant backups and protect your systems against data loss in the event that one drive fails. Learn some basic RAID management.

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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 updated.

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.