Playing around with software raid

And you don’t even need any physical disks for it….

I don’t use Linux software raid tool mdadm that often so I quickly forget how it works. This is something I used on several occasions, when trying to refresh my mind. The cool thing is that you don’t need physical disks or a lot of space for it to work. The following might vary a bit depending on you system (mine is Ubuntu Edgy Eft on IBM x40).

First create a few “disks”… by creating some empty files and making them into block devices:

dd if=/dev/zero of=disk1 bs=1M count=1 seek=30
dd if=/dev/zero of=disk2 bs=1M count=1 seek=30
dd if=/dev/zero of=disk3 bs=1M count=1 seek=30
losetup /dev/loop0 disk1
losetup /dev/loop1 disk2
losetup /dev/loop2 disk3

This creates 3 files (disk1, disk2 and disk3) with the size of 1MB in the current directory and makes them into block devices (just like normal disks is).

Now create your raid, example:

mdadm --create /dev/md0 --level=5 --raid-devices=2 --spare-devices=1 /dev/loop0 /dev/loop1 /dev/loop2

If you get the error:

mdadm: error opening /dev/md0: No such file or directory

Add the parameter --auto=md to the raid create command.

Now you can see you raid status with:

cat /proc/mdstat

Now play around with it all you want


When you are done you stop the raid and remove it with the following:

mdadm --stop /dev/md0
mdadm --remove /dev/md0

Perhapes you want to remove the md0 device again with (only if you needed the --auto=md parameter:

rm /dev/md0

Cleanup the “disks”:

losetup -d /dev/loop2
losetup -d /dev/loop1
losetup -d /dev/loop0
rm disk3
rm disk2
rm disk1

Now you computer wont have a trace of you your software raid disks… besides you shell history :)

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Playing around with software raid by Jacob Emcken is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.