Is it possible to shrink KVM virtual machine LVM partitioned disk? Well, to be honest, this is an almost mission impossible task, but it is doable. Shrinking a disk may nuke your disk and cause a total loss of the data. Therefore, this is not an official guide on how to shrink a partition on KVM virtual machines, but rather risky steps i took to get my vm disk down. Be cautious!
Table of Contents
Shrinking KVM Virtual Machine LVM Partitioned Disk
In this tutorial, we want to an LVM partitioned disk on a KVM virtual machine;
fdisk -l /dev/sda
Disk /dev/sda: 20 GiB, 21474836480 bytes, 41943040 sectors
Disk model: QEMU HARDDISK
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x75b85259
Device Boot Start End Sectors Size Id Type
/dev/sda1 * 2048 999423 997376 487M 83 Linux
/dev/sda2 1001470 41940991 40939522 19.5G 5 Extended
/dev/sda5 1001472 41940991 40939520 19.5G 8e Linux LVM
lvs
LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert
root bookworm-vg -wi-ao---- 18.56g
swap_1 bookworm-vg -wi-ao---- 980.00m
And this is how these partitions are mounted;
lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
sda 8:0 0 20G 0 disk
├─sda1 8:1 0 487M 0 part /boot
├─sda2 8:2 0 1K 0 part
└─sda5 8:5 0 19.5G 0 part
├─bookworm--vg-root 254:0 0 18.6G 0 lvm /
└─bookworm--vg-swap_1 254:1 0 980M 0 lvm [SWAP]
Update FSTAB to use Device Instead of UUID
As mentioned before, configure FSTAB to use device instead of UUID. When you shrink the partition, UUID may change and you wont be able to boot the virtual machine.
To get the device for each mapping, use the command below;
blkidSample output;
/dev/mapper/bookworm--vg-swap_1: UUID="9e491bc0-7d36-4f73-9692-dc1dacde28d3" TYPE="swap"
/dev/mapper/bookworm--vg-root: UUID="b449ef15-3467-4b59-b881-d7e15d385210" BLOCK_SIZE="4096" TYPE="ext4"
/dev/sda5: UUID="JK72C1-ginB-1kph-t96A-NV4c-8H3e-6wxgpc" TYPE="LVM2_member" PARTUUID="75b85259-05"
/dev/sda1: UUID="ef714d79-6130-4ff8-8eba-1bac9303846c" BLOCK_SIZE="1024" TYPE="ext2" PARTUUID="75b85259-01"
Hence update the filesystem table;
sudo vim /etc/fstab
...
#
/dev/mapper/bookworm--vg-root / ext4 errors=remount-ro 0 1
# /boot was on /dev/sda1 during installation
UUID=ef714d79-6130-4ff8-8eba-1bac9303846c /boot ext2 defaults 0 2
/dev/mapper/bookworm--vg-swap_1 none swap sw 0 0
We can updat the file like as shown below;
...
#
/dev/mapper/bookworm--vg-root / ext4 errors=remount-ro 0 1
# /boot was on /dev/sda1 during installation
#UUID=ef714d79-6130-4ff8-8eba-1bac9303846c /boot ext2 defaults 0 2
/dev/sda1 /boot ext2 defaults 0 2
#/dev/mapper/bookworm--vg-swap_1 none swap sw 0 0
Save the file and exit.
Note that I have also commented the SWAP partition line to avoid being mounted. I dont really need swap for now. This can easily be added later;
In fact, let’s remove the swap logival volume.
Find out the swap space device;
swaponSample output;
NAME TYPE SIZE USED PRIO
/dev/dm-1 partition 980M 0B -2
Turn off the swap;
swapoff /dev/dm-1Deactivate the Swap logical volume;
lvchange -an /dev/bookworm-vg/swap_1and remove it;
lvremove /dev/bookworm-vg/swap_1Shrink the Physical Volume and Filesystem
To shrink a physcal volume, follow the guide below;
How to Reduce or Shrink Physical Volume in Linux
Shrink the Disk Size
After you have resized the logical partitions above, the disk size still hasn’t changed.
fdisk -l /dev/sda
Disk /dev/sda: 20 GiB, 21474836480 bytes, 41943040 sectors
Disk model: QEMU HARDDISK
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x75b85259
Device Boot Start End Sectors Size Id Type
/dev/sda1 * 2048 999423 997376 487M 83 Linux
/dev/sda2 999424 21979135 20979712 10G 5 Extended
/dev/sda5 1001472 21979135 20977664 10G 83 Linux
You can see above,
So, to shrink the disk, you need to power off the virtual machine;
virsh shutdown <vm-name>Or just poweroff from the virt-manager.
Next, check the Disk information;
qemu-img info /media/kifarunix/vol02/kvm/images/bookworm.qcow2
image: /media/kifarunix/vol02/kvm/images/bookworm.qcow2
file format: qcow2
virtual size: 20 GiB (21474836480 bytes)
disk size: 2.14 GiB
cluster_size: 65536
Format specific information:
compat: 1.1
lazy refcounts: true
refcount bits: 16
corrupt: false
As you can see, disk is still showing 20G. Let’s shrink it.
NOTE that we are shrinking it to 10244MiB. In order to be in safe situation, ensure that the size is a bit higher, for example, let’s set the size to be +512MiB more, hence, size of 10756MiB;
qemu-img resize --shrink /media/kifarunix/vol02/kvm/images/bookworm.qcow2 10756MCheck the information again;
qemu-img info /media/kifarunix/vol02/kvm/images/bookworm.qcow2
image: /media/kifarunix/vol02/kvm/images/bookworm.qcow2
file format: qcow2
virtual size: 10.5 GiB (11278483456 bytes)
disk size: 1.92 GiB
cluster_size: 65536
Format specific information:
compat: 1.1
lazy refcounts: true
refcount bits: 16
corrupt: false
Confirm Disk Shrinkage on the System
Boot the machine and confirm that it boots and the disk is now shrinked!
fdisk -l /dev/sda
Disk /dev/sda: 10.5 GiB, 11278483456 bytes, 22028288 sectors
Disk model: QEMU HARDDISK
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x75b85259
Device Boot Start End Sectors Size Id Type
/dev/sda1 * 2048 999423 997376 487M 83 Linux
/dev/sda2 999424 21979135 20979712 10G 5 Extended
/dev/sda5 1001472 21979135 20977664 10G 83 Linux
Looks like it! Phew!
And that is all on shrinking KVM virtual machine LVM partitioned disk.
