How To Extend KVM Virtual Machine Disk Size In Linux

This brief guide explains what is Virt-size command line tool and how can we expand or extend KVM virtual machine disk size using Virt-resize tool in Linux.

The other day I noticed that one of my KVM Virtual machine is running out of disk space. I couldn't install any new applications or couldn't save files due to low disk space. To fix this issue, I simply increased the size of a KVM virtual machine's disk using virt-size command. I personally tested the steps given below to increase the disk size of a CentOS 8 KVM virtual machine and it worked as expected.

Virt-resize tool

Virt-resize is a command line tool to resize a virtual machine disk. Using virt-resize, we can either increase or decrease the size of a virtual machine's disk. We can also delete the partitions inside the virtual disk with virt-resize command. Please note that virt-resize the live machines. The guest systems should be turned off before resizing their disk images.

To expand a Virtual machine's disk size, we first create a larger disk image than the source image. And then we copy the source image to the destination image and finally resize it.

Virt-resize is the part of the libguestfs-tools package. To install libguestfs-tools package on Debian, Ubuntu and its derivatives, run:

$ sudo apt install libguestfs-tools

To install libguestfs-tools package on Fedora, RHEL and its clones, run:

$ sudo dnf install libguestfs-tools

In Fedora 34, guestfs-tools package provides virt-resize tool. So if you're on Fedora 34, install guestfs-tools package like below:

$ sudo dnf install guestfs-tools

Now let us see how to expand virtual machine disk size using virt-resize command.

Expand or extend KVM virtual machine disk size in Linux

For the purpose of this guide, I will be using CentOS 8 virtual machine disk image named centos8.qcow2. I have stored this mage in my current working directory. You can use virsh dumpxml command to locate the disk image that you want to resize.

Example:

$ virsh dumpxml centos8

Replace centos8 with your VM name in the above command.

After locating the disk image you want to resize, inspect the partition details inside this disk image. To print the partition details and their size of a disk image, run:

$ virt-filesystems --long --parts --blkdevs -h -a centos8.qcow2 

Sample output:

 Name       Type       MBR  Size  Parent
 /dev/sda1  partition  83   1.0G  /dev/sda
 /dev/sda2  partition  8e   19G   /dev/sda
 /dev/sda   device     -    20G   -

As you can see in the above output, my disk image has only two partitions, namely /dev/sda1 with size 1 GB and /dev/sda2 with 19GB. The total size of disk image is 20 GB.

Now we will,

• increase the total disk image size from 20 GB to 30 GB,
• increase the size of the first partition /dev/sda1 from 1.0GB to 1.5GB,
• Expand the second partition /dev/sda2 to fill the remaining space.

Virt-resize cannot resize disk images in-place. Also, Virt-resize should not be used on live virtual machines. Before resizing disk image, power off the Virtual machine and take the backup of the original disk image.

$ mv centos8.qcow2 centos8.qcow2.backup

The above command will rename current disk image called centos8.qcow2 to centos8.qcow2.backup.

Create a new disk image with same name and with size 30GB using command:

$ qemu-img create -f qcow2 -o preallocation=metadata centos8.qcow2 30G

Sample output:

Formatting 'centos8.qcow2', fmt=qcow2 cluster_size=65536 extended_l2=off preallocation=metadata compression_type=zlib size=32212254720 lazy_refcounts=off refcount_bits=16

Make sure the size of the newly created disk is 30GB:

$ virt-filesystems --long --parts --blkdevs -h -a centos8.qcow2

Sample output:

Name      Type    MBR  Size  Parent
 /dev/sda  device  -    30G   -

Now expand/extend the KVM virtual machine disk image size using command:

$ virt-resize --resize /dev/sda1=+500M --expand /dev/sda2 centos8.qcow2.backup centos8.qcow2

Here,

• --resize /dev/sda1=+500M - increases the size of the /dev/sda1 partition to 500 MB.
• --expand /dev/sda2 - resizes the /dev/sda2 partition to to fill all remaining space. In my case, it will be resized from 19.0G to 28.5G.
• centos8.qcow2.backup - it is the original input disk image.
• centos8.qcow2 - it is the newly created output disk image.

Sample output of the above command is:

[   0.0] Examining centos8.qcow2.backup
 
 Summary of changes:
 /dev/sda1: This partition will be resized from 1.0G to 1.5G.  The 
 filesystem ext4 on /dev/sda1 will be expanded using the ‘resize2fs’ 
 method.
 /dev/sda2: This partition will be resized from 19.0G to 28.5G.  The LVM PV 
 on /dev/sda2 will be expanded using the ‘pvresize’ method.
 
 [   4.2] Setting up initial partition table on centos8.qcow2
 [   5.6] Copying /dev/sda1
  100% ⟦▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒⟧ 00:00
 [   9.7] Copying /dev/sda2
  100% ⟦▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒⟧ 00:00
 [  45.1] Expanding /dev/sda1 using the ‘resize2fs’ method
 [  45.3] Expanding /dev/sda2 using the ‘pvresize’ method
 Resize operation completed with no errors.  Before deleting the old disk, 
 carefully check that the resized disk boots and works correctly.

Check the partitions and their sizes with virt-filesystems command:

$ virt-filesystems --long -h --all -a centos8.qcow2

Sample output:

 Name          Type        VFS   Label  MBR  Size  Parent
 /dev/sda1     filesystem  ext4  -      -    1.4G  -
 /dev/cl/root  filesystem  xfs   -      -    17G   -
 /dev/cl/swap  filesystem  swap  -      -    2.0G  -
 /dev/cl/root  lv          -     -      -    17G   /dev/cl
 /dev/cl/swap  lv          -     -      -    2.0G  /dev/cl
 /dev/cl       vg          -     -      -    29G   /dev/sda2
 /dev/sda2     pv          -     -      -    29G   -
 /dev/sda1     partition   -     -      83   1.5G  /dev/sda
 /dev/sda2     partition   -     -      8e   29G   /dev/sda
 /dev/sda      device      -     -      -    30G   -

As you see in the above output, my CentOS8 KVM disk image size has been resized from 20 GB to 30 GB.

Now boot the virtual machine with new disk image. You can also create a new virtual machine using the Qcow2 image as described in the following link.

• Create A KVM Virtual Machine Using Qcow2 Image In Linux

If it works without any issues, then thoroughly inspect if all your data exists. If everything is OK, delete the old backup disk.

If there are any issues with new disk, shutdown the virtual machine and rename backup disk to its original name. Repeat the above steps carefully again. Don't forget to backup the data and disk image.

Resource:

• Virt-resize website

Related read:

• Access And Modify Virtual Machine Disk Images With Libguestfs
• How To Rescue Virtual Machines With Virt-rescue