How to move /usr to a new partition?

My /usr folder needs to get moved to a new partition. How can I do this without erasing the contents?

Can this be done while Ubuntu is running, or do I need to use the LiveCD for this?

It would be safest to use a Live CD, but you could do:

• copy all the files to a new partition, making sure that the contents of /usr do not change while you are doing this.
• edit /etc/fstab so that /usr will be mounted on the next reboot
• reboot
• delete the old files

See below for details on each step.

Note that you can't mount the new partition on /usr while running as there will be lots of files in /usr that will be open.

Copying the files

I would use cp -a. -a is the archive option. From the man page:

-a, --archive
          same as -dR --preserve=all
...
-d     same as --no-dereference --preserve=links
...
-P, --no-dereference
          never follow symbolic links in SOURCE
...
--preserve[=ATTR_LIST]
          preserve      the      specified      attributes       (default:
          mode,ownership,timestamps),  if  possible additional attributes:
          context, links, xattr, all
...
-R, -r, --recursive
          copy directories recursively

Editing /etc/fstab

You need to know the UUID of your new partition. You can see the mapping by doing:

$ ls -l /dev/disk/by-uuid/

And then add this line to /etc/fstab:

UUID=634c31a5-e27c-4e33-ac67-2e22491a30c2 /usr           ext4    defaults        0       2

Change the UUID to your UUID, and change ext4 to be the file system type you are using - you should know this if you have set up the partition.

Delete the old files

After the reboot, the old files in /usr on the root partition will be hidden by the new partition mounted on /usr. But we can use some mount bind trickery to get to the old files and then delete them.

$ sudo mount --bind / /mnt
$ sudo rm -rf /mnt/usr/*
$ sudo umount /mnt

But some slight mistyping (say, hitting Enter when you'd only typed sudo rm -rf /mnt ) could cause disaster, so I would only use this method if you were very confident in what you were doing, really couldn't deal with any downtime, or had no physical access to the machine and hence were unable to boot off a live CD or live USB stick.

Since most libraries that are used are in /usr, I would not recommend to move this directory while running Ubuntu. In fact, you probably get error messages when you try to do this. Hence, the best is to use the LiveCD.

You can use several possibilities to move/copy the files cp, rsync etc. you want to make sure that any symlinks are created and not just copied. cp and rsync both have options for this.

After moving the files to the other partition you need to add another mount in /etc/fstab to mount the new partition to /usr.

This is how I've done it (following the Hamish's answer and the comments):

1. Copy all the files the newly created partition (replace with the location of your partition, it should look similar to mine):

    rsync -avz /usr/* /media/aleksandar/750b84e2-e65f-4309-ade5-5af0033a937c 
   

2. Edit /etc/fstab (same as in Hamish's answer, of course, replace xxxxxx with your UUID)

    UUID=xxxxxx/usr           ext4    defaults        0       2
   

3. Reboot the system

4. After rebooting, open System Monitor or similar application to see whether your new /usr partition is mounted, and to safety-check whether everything went as planned.

5. After checking that everything is alright, you can delete your old /usr partition. I will keep mine just in case something goes wrong.