LVM

HowTo - shrink and resize LUKS-encrypted LVM volumes

LVM and LUKS are often used together within a standard encrypted setup. Sometimes we need to to e.g. shrink an existing Ubuntu installation to fit on another disk (move from bigger hdd to a smaller sdd) or vice versa.

Shrinking is not such a big task. For most people the part of actually shrinking the partition is the most scary part! But, if you just calculate using a few GB of buffer size (which is really not a big thing), you don't have to deal with calculations of exact sector of offset sizes in fdisk or parted.

The best howtos I could find for Ubuntu which really sums up the essential tasks in a very good manner is on Ubuntuforums and on Archwiki. I mainly follow the Ubuntuforum article but when it comes to finally resize the disk partition, the archwiki is more instructable using parted (in my opinion). Both describe the procedure using a life disk. If you are using you PC which is also encrypted you should read the LVM#Troubleshooting section.

• ubuntuforums.org thread (3/2008): How to Resize a LUKS Encrypted File System
• Arch-Wiki: REsizing LVM on LUKS

Install necessary apps

E.g. on a rescue disc ...

apt-get install cryptsetup

Open encryted lvm partition

Be arefulat this step!

If this is the root device, you will need to use exactly the _ SAME NAME _ for the crypt (in our example: sda5_crypt for ${my_crpyt_name}) like in your destination Ubuntu environment. If not, you will not be able to boot your device, because the system will be configured using the wrong name for your mapper name which is written down in /etc/crypttab (don't change this!).

# Please edit acccording to your entry in /etc/crypttab of your destination installation
# If you don't now it yet, see the troubleshooting section next.
my_crypt_name=sda5_crypt
cryptsetup luksOpen /dev/sda5 ${my_crpyt_name}

Troubleshooting

Name of the cryptdisk is important

If you try to encrypt a system from within an encrypted system, which uses the _SAME_ crypt name as the destination system this will _NOT WORK_. Then the only way is to use a life cd or another PC with an uncrypted installation !!!

If you are unsure about the name of ${my_crpyt_name} which is used within your destination setup, you have to look into its /etc/crypttab. To to this: decrypt your device, mount it, have a look into the /etc/crypttab, unmount, uncrypt and start over again with the right name:

cryptsetup luksOpen /dev/sda5 test_crypt
mount /dev/mapper/vg-somename-root /mnt/test

nano /mnt/test/etc/crypttab
> sda5_crypt UUID=def346a0-6e33-4523-b99c-d7777b980b34 none luks,discard

umount /mnt/test
crpytsetup luksClose test_crypt

Resizing the disk partition table with tools like fdisk or parted.

Actually resizing the partition does not hurt anything on the filessystem! It just marks the space for the So be not afraid! Normally the partition table of an encrypted LUKS linux system looks like this:

/dev/sdx1          5G        83 Linux (this is /boot)           
/dev/sdx2          115G       5 Extented
 └─/dev/sdd5       115G      83 Linux

What we - after shrinking - want to achieve is something like this:

/dev/sdx1          5G        83 Linux (this is /boot)           
/dev/sdx2          75G        5 Extented
 └─/dev/sdd5       75G       83 Linux
Free space         40G

I am using parted (as shown in link to the the Arch-Wiki article above), but you also can use fdisk to do this. I just do the following:

• resize to the minimal Size (in GB)

parted
  resizepart 5
  [120GB]? 80GB
  Warning ...
  Yes? Yes 

  resizepart 2
  [120GB]? 80GB
  Warning ...
  Yes? Yes 

• remount cryptdisk (luksOpen)
• scan for the volumes (vgchange -ay)
• check the filesystem (e2fsck -f /dev/mapper/volume-root

If something goes wrong here:

• vgchange -an (get rid of volumes)
• luksClose ...
• parted: give a few more space (often 1-2GB are enough)
• mount and recheck as shown above

Mount existing volume groups

sudo apt-get install lvm2
sudo modprobe dm-mod

sudo vgchange -a y
> 2 logical volume(s) in volume group "vg-whatever" now active

ls /dev/mapper
> control sda5_crypt vg--whatever--root  vg--whatever--swap

# Please edit
my_root=/mnt/whatever_root
my_vg=vg--whatever--root
my_boot_device=/dev/sda1

# Mount /root and /boot
mkdir ${my_root}
mount /dev/mapper/${my_vg} ${my_root}
mount ${my_boot_device} ${my_root}/boot

# Chroot
mount -o bind /dev ${my_root}/dev; \
mount -o bind /run ${my_root}/run; \
mount -t proc /proc ${my_root}/proc; \
mount -t sysfs /sys ${my_root}/sys
chroot ${my_root}

Umount existing volume groups and close encrypted container

sudo umount ${my_root}/*
sudo umount /dev/mapper/${my_vg}
sudo vgchange -a n
sudo cryptsetup luksClose ${my_crpyt}