vgdisplay

Resizing ext3-on-LVM-on-dmcrypt file systems, moving disk space from one LV to another

Back in 2008 I wrote a small article about resizing LVM physical volumes. I had to do something similar, but slighly more complicated, recently. My /usr logical volume (LV) was getting full on my laptop disk, thus I wanted to shrink another LV and move some of that space to /usr. Here's one way you can do that.

Requirements: a Live CD containing all required utilities (cryptsetup, LVM tools, resize2fs), I used grml.

Important: If you plan to perform any of these steps, make sure you have recent backups! I take no responsibility for any data loss you might experience. You have been warned!

First, shutdown the laptop and boot using the Live CD. Then, open the dm-crypt device (/dev/hda3 in my case) by entering your passphrase:

  $ cryptsetup luksOpen /dev/hda3 foo

Activate all (newly available) LVM volume groups in that encrypted device:

  $ vgchange -a y

(maybe you also need a vgscan and/or lvscan, not sure)

Check how much free space we have for putting into our /usr LV:

  $ vgdisplay | grep Free
  Free  PE / Size       0 / 0   

OK, so we have none. Thus, we need to shrink another LV (/home, in my case) and put that newly freed space into the /usr LV. In order to do that, we have to check the current size of the /home LV:

  $ mount -t ext3 /dev/vg-whole/lv-home /mnt
  $ df --block-size=1M | grep -C 1 /mnt
  $ umount /mnt

(if you know how to find out the size of an ext3 file system without mounting it, please let me know) Update: See comments for suggestions.

Write down the total amount of 1M chunks of space on the file system (116857 in my case), we'll need that later. Now run 'fsck' on the /home LVM logical volume, which is needed for the 'resize2fs' step afterwards. This will take quite a while.

  $ fsck -f /dev/vg-whole/lv-home

Next step is resizing the ext3 file system in the /home LVM logical volume, making it 1GB smaller than before (of course you must have >= 1 GB of free space on /home for that to work). We use fancy bash calculations to do the math.

Note: I'm not so sure about the sizes here, in my first attempt something went wrong and resize2fs said "filesystem too small" or the like. Maybe I'm confusing the size units from 'df' and 'resize2fs', or the bash calculation goes wrong? Please leave a comment if you know more!

  $ resize2fs /dev/vg-whole/lv-home $((116857-1024))M

Then, we can safely reduce the LV itself. Note: order is very important here, you must shrink the ext3 filesystem first, and then shrink the LV! Doing it the other way around will destroy your filesystem!

  $ lvreduce -L -1G /dev/vg-whole/lv-home

Now that we have 1 GB of free space to spend on LVs, we assign that space to the /usr LVM logical volume like this:

  $ lvextend -L +1G /dev/vg-whole/lv-usr

As usual, we then run 'fsck' on the filesystem in order to be able to use 'resize2fs' to resize it to the biggest possible size (that's the default if resize2fs gets no parameters):

  $ fsck -f /dev/vg-whole/lv-usr
  $ resize2fs /dev/vg-whole/lv-usr

That's it. You can now shutdown the Live CD system and boot into the normal OS with the new space allocations:

  $ vgchange -a n
  $ cryptsetup luksClose foo
  $ halt
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