LVM Resize – reduce the size of one logical volume to enable expanding another

Posted on by rchapin

I’m running an Ubuntu workstation and when setting it up simply went the “next-next-next” route when setting up the encrypted disk via LVM. The default is to create a 1G swap partition which is just not enough when you attempt to run too many things and locks up and/or crashes the machine.

My goal was to reduce my /root partition and then use that space to extend my swap partition.

Ensure that you back up your data first! There is no guarantee when executing the following operations, even correctly, that you will not lose your data.

Decreasing the size of an LVM partition

Because I need to modify the /root partition I first needed to boot to a live image or rescue image. I happened to have a Debian 12 iso on a flash drive and after sorting out how to get my laptop to boot from it chose the rescue option when booting.

One of the unexpected options during boot to the rescue image was to decrypt the filesystem so I simply entered the passphase to decrypt it. If you are using a rescue image that does not include that feature checkout this post for details on how to decrypt it.

An overview of the steps is as follows

  1. Run a filesystem check on your filesystem
  2. Resize the filesystem contained in the logical volume
  3. Resize the logical volume

Run a filesystem check

Since you have booted from a rescue disk you first need to activate the volume group to work with it

vgchange -ay

Enter the following to the details for the volume group

root@marge:~# vgdisplay
  --- Volume group ---
  VG Name               marge-vg
  System ID             
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  3
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                2
  Open LV               2
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               <465.27 GiB
  PE Size               4.00 MiB
  Total PE              119108
  Alloc PE / Size       119108 / <465.27 GiB
  Free  PE / Size       0 / 0   
  VG UUID               m06QqY-lQ3N-Y0be-j7nw-V1jo-hHwK-1OrE6U

Then use lvdisplay to see the details of all of the logical volumes contained within that group

root@marge:~# lvdisplay
  --- Logical volume ---
  LV Path                /dev/marge-vg/root
  LV Name                root
  VG Name                marge-vg
  LV UUID                820Zm6-zOcV-9gMu-efCr-jnVv-nmzX-T04zFq
  LV Write Access        read/write
  LV Creation host, time marge, 2022-12-15 14:58:30 -0500
  LV Status              available
  # open                 1
  LV Size                <464.31 GiB
  Current LE             118863
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:1
   
  --- Logical volume ---
  LV Path                /dev/marge-vg/swap_1
  LV Name                swap_1
  VG Name                marge-vg
  LV UUID                MHyNcx-ASX7-bsDd-Wto3-k9u8-QjZA-Z85yIR
  LV Write Access        read/write
  LV Creation host, time marge, 2022-12-15 14:58:30 -0500
  LV Status              available
  # open                 2
  LV Size                980.00 MiB
  Current LE             245
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:2

Run the filesystem check as you cannot resize a filesystem that in a bad state. The LV Path is the path to the underlying file system.

e2fsck -fy /dev/marge-vg/root

Shrink the filesystem

Just to be safe, we will shrink the filesystem to be smaller than the target logical volume size. If we accidentally shrink the logical volume to be smaller than the filesystem that it contains this can result in corruption of your data. Once the operation is complete, we can reclaim that “lost” space. The last argument in the command specifies the desired target size of the filesystem.

resize2fs /dev/marge-vg/root 430G

Shrink the logical volume

Once the filesystem is shrunk we can now shrink the logical volume. Again, we will specify a specific target size for the logical volume of 434G. You will get a warning that the operation could result in data loss. If we have performed the previous steps successfully we are (relatively) safe to proceed.

lvreduce -L 434G /dev/marge-vg/root

lvdisplay should now show the smaller logical volume

root@marge:~# lvdisplay
  --- Logical volume ---
  LV Path                /dev/marge-vg/root
  LV Name                root
  VG Name                marge-vg
  LV UUID                820Zm6-zOcV-9gMu-efCr-jnVv-nmzX-T04zFq
  LV Write Access        read/write
  LV Creation host, time marge, 2022-12-15 14:58:30 -0500
  LV Status              available
  # open                 1
  LV Size                <434.00 GiB  <-- New LV Size
  Current LE             118863
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:1

Now that the logical volume has been reduced we can reclaim the additional space (the difference between the reduced filesystem size and the new logical volume size) by extending the filesystem to use all available space in the logical volume.

resize2fs /dev/marge-vg/root

Extend the other logical volume

Verify the amount of free space now available in the volume group

root@marge:~# vgdisplay
  --- Volume group ---
  VG Name               marge-vg
  System ID             
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  3
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                2
  Open LV               2
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               <465.27 GiB
  PE Size               4.00 MiB
  Total PE              119108
  Alloc PE / Size       111427 / <435.27 GiB
  Free  PE / Size       7618 / 30.00 GiB  <-- Free space
  VG UUID               m06QqY-lQ3N-Y0be-j7nw-V1jo-hHwK-1OrE6U

Run the following command to extend the logical volume. In this case, we will extend the swap volume by 30 G

lvextend -L +30G /dev/marge-vg/swap_1

Resize the filesystem for the extended logical volume

We have only resized the logical volume. The filesystem contained therein has not yet been resized to use the additional space

Because we are resizing a swap partition we need to do things a little differently. Run the following to recreate the swap partition in this logical volume

mkswap /dev/marge-vg/swap_1

For a “normal” partition you would run the following to extend the filesystem to the size of the logical volume.

resize2fs /dev/<vol-group>/<lv>

From here you should be able to reboot your system with your new LVM configuration.

Posted in LVM

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