mdadm is a software RAID technology for creating local volumes with RAID functionality such as RAID 5. RAID 5 is used to provide redundancy in the even of a disk failure by using the equivalent space of one disk in the volume for parity data.

One advantage of using mdadm is that you can create volumes of disks to a given size and then expand them at a later date. For example, if you have a RAID 5 array with 3 disks you can expand it to four disks.

Before following these steps you should un-mount the file system using umount.

This example assumes your array is called md1 and the new disk is sdf.

mdadm --add /dev/md1 /dev/sdf1

Now we can grow /dev/md1 as follows.

mdadm --grow /dev/md1 --size=max

If this fails, you may need to remove the bitmap index before retrying the above command.

mdadm --grow --bitmap=none /dev/md1

The process will now start which can take a while. An Example, 5 x 2TB WD green disks took 3 days to complete. You can view the progress with:

watch cat /proc/mdstat

Once this completes, we run a file system check and resize the file system.

e2fsck -f /dev/md1
resize2fs /dev/md1
e2fsck -f /dev/md1

You can now re-mount your file system.

mount -a