How do you increase a KVM guest's disk space?

1. stop the VM
2. run qemu-img resize vmdisk.img +10G to increase image size by 10Gb
3. start the VM, resize the partitions and LVM structure within it normally

Online Method (using qemu, libvirt, and virtio-block)

For better or worse, the commands below will run even if the target virtual disk is mounted. This can be useful in environments where the disk cannot be unmounted (such as a root partition), the VM must stay on, and the system owner is willing to assume the risk of data corruption. To remove that risk, you would need to log into the VM and unmount the target disk first, something that isn't always possible.

Perform the following from the KVM hypervisor.

1. Increase the size of the disk image file itself (specify the amount to increase):

   qemu-img resize <my_vm>.img +10G
   

2. Get the name of the virtio device, via the libvirt shell (drive-virtio-disk0 in this example):

   virsh qemu-monitor-command <my_vm> info block --hmp
     drive-virtio-disk0: removable=0 io-status=ok file=/var/lib/libvirt/images/<my_vm>.img ro=0 drv=raw encrypted=0
     drive-ide0-1-0: removable=1 locked=0 tray-open=0 io-status=ok [not inserted]
   

3. Signal the virtio driver to detect the new size (specify the total new capacity):

   virsh qemu-monitor-command <my_vm> block_resize drive-virtio-disk0 20G --hmp
   

Then log into the VM. Running dmesg should report that the virtio disk detected a capacity change. At this point, go ahead and resize your partitions and LVM structure as needed.

These serverfault questions are similar but more specific, KVM online disk resize? & Centos Xen resizing DomU partition and volume group. The 1st asks the question of how to increase a KVM guest while it's online, while the 2nd is XEN specific using LVM. I'm asking how to accomplish this while the KVM is offline.

NOTE: This link was useful for METHOD #1, and shows how to accomplish increasing a KVM's disk space (ext3 based), HOWTO: Resize a KVM Virtual Machine Image.

One thing to be aware of with KVM guests is that the partitions they're using inside can effect which method you can use to increase their disk space.

METHOD #1: Partitions are ext2/ext3/ext4 based

The nuts of this method are as follows:

# 1. stop the VM
# 2. move the current image
mv mykvm.img mykvm.img.bak

# 3. create a new image
qemu-img create -f raw addon.raw 30G

# 4. concatenate the 2 images
cat mykvm.img.bak addon.raw >> mykvm.img

Now with the larger mykvm.img file in hand, boot gparted and extend the existing partition into the newly added disk space. This last step basically extends the OS partition so that it can make use of the extra space.

METHOD #2: Partitions are LVM based

Here are the steps that I roughly followed to resize a KVM guest that used LVM internally.

1. Shutdown the VM
2. add more space to the guest's "image file" (something like: cat old.img 10G_addon.raw >> new.img
3. start the VM (using the newly created new.img)

4. run fdisk inside VM and delete & re-create LVM partition

   % fdisk /dev/vda
   ...
   Device Boot      Start         End      Blocks   Id  System
   /dev/vda1   *           1          13      104391   83  Linux
   /dev/vda2              14        3263    26105625   8e  Linux LVM
   
   Command (m for help): d
   Partition number (1-4): 2
   
   Command (m for help): p
   
   Disk /dev/vda: 48.3 GB, 48318382080 bytes
   255 heads, 63 sectors/track, 5874 cylinders
   Units = cylinders of 16065 * 512 = 8225280 bytes
   
   Device Boot      Start         End      Blocks   Id  System
   /dev/vda1   *           1          13      104391   83  Linux
   
   Command (m for help): n 
   Command action
     e   extended
     p   primary partition (1-4)
   p
   Partition number (1-4): 2
   First cylinder (14-5874, default 14): 14
   Last cylinder or +size or +sizeM or +sizeK (14-5874, default 5874): 
   Using default value 5874
   
   Command (m for help): p
   
   Disk /dev/vda: 48.3 GB, 48318382080 bytes
   255 heads, 63 sectors/track, 5874 cylinders
   Units = cylinders of 16065 * 512 = 8225280 bytes
   
   Device Boot      Start         End      Blocks   Id  System
   /dev/vda1   *           1          13      104391   83  Linux
   /dev/vda2              14        5874    47078482+  83  Linux
   
   Command (m for help): t
   Partition number (1-4): 2
   Hex code (type L to list codes): 8e
   Changed system type of partition 2 to 8e (Linux LVM)
   
   Command (m for help): p
   
   Disk /dev/vda: 48.3 GB, 48318382080 bytes
   255 heads, 63 sectors/track, 5874 cylinders
   Units = cylinders of 16065 * 512 = 8225280 bytes
   
   Device Boot      Start         End      Blocks   Id  System
   /dev/vda1   *           1          13      104391   83  Linux
   /dev/vda2              14        5874    47078482+  8e  Linux LVM
   
   Command (m for help): w
   The partition table has been altered!
   
   Calling ioctl() to re-read partition table.
   
   WARNING: Re-reading the partition table failed with error 16: Device or 
   resource busy.
   The kernel still uses the old table.
   The new table will be used at the next reboot.
   Syncing disks.
   %
   

5. Reboot the VM

6. Resize the LVM physical volume

   % pvdisplay 
     --- Physical volume ---
     PV Name               /dev/vda2
     VG Name               VolGroup00
     PV Size               24.90 GB / not usable 21.59 MB
     Allocatable           yes (but full)
     PE Size (KByte)       32768
     Total PE              796
     Free PE               0
     ...
   
   % pvresize /dev/vda2
   
   % pvdisplay
     --- Physical volume ---
     PV Name               /dev/vda2
     VG Name               VolGroup00
     PV Size               44.90 GB / not usable 22.89 MB
     Allocatable           yes 
     PE Size (KByte)       32768
     Total PE              1436
     Free PE               640
     ...
   

7. Resize the LVM Logical Volume

     % lvresize /dev/VolGroup00/LogVol00 -l +640
     Extending logical volume LogVol00 to 43.88 GB
     Logical volume LogVol00 successfully resized
   

8. Grow the File system

     % resize2fs /dev/VolGroup00/LogVol00 
     resize2fs 1.39 (29-May-2006)
     Filesystem at /dev/VolGroup00/LogVol00 is mounted on /; on-line resizing required
     Performing an on-line resize of /dev/VolGroup00/LogVol00 to 11501568 (4k) blocks.
     The filesystem on /dev/VolGroup00/LogVol00 is now 11501568 blocks long.
   

The above is my example, but I followed the steps on this website

Resize and Expand Internal Partitions in One Step

I had an Ubuntu host with a qcow2 guest file image and wanted to resize the disk and expand the appropriate partitions all in one step. It requires you to set up the libvirt guest filesystem utilities, but those are useful to have around anyway.

Inspiration from here: http://libguestfs.org/virt-resize.1.html

The key command here is: virt-resize

• this is a libvirt utility
• it can expand a guest disk AND expand the partitions inside in one step
• it copies the disk, it doesn't expand it in-place
  • this is good because you have the untouched original as a backup

Preparation:

* Install libvirt file system utilities package 
    * sudo apt-get install libguestfs-tools

* Test to see if it works (it won't) -- you need to see "===== TEST FINISHED OK =====" at the bottom:
    * sudo libguestfs-test-tool

* If you don't see "===== TEST FINISHED OK =====" at the bottom then repair it:
    * sudo update-guestfs-appliance

* Run the test again and verify it works
    * sudo libguestfs-test-tool

Now do the following:

1) shutdown the guest:

2) Check out the current sizing and view the partition name you want to expand using libvirt utility:

sudo virt-filesystems --long --parts --blkdevs -h -a name-of-guest-disk-file

3) Create the new (40G) output disk:

qcow: sudo qemu-img create -f qcow2 -o preallocation=metadata outdisk 40G
img:  sudo truncate -s 40G outdisk

4) Copy the old to the new while expand the appropriate partition (assuming your disk partition from step 2 was /dev/sda1):

sudo virt-resize --expand /dev/sda1 indisk outdisk

5) Rename the indisk file as a backup, rename the outdisk as indisk (or modify the guest XML)

6) Reboot the guest and test the new disk file carefully before deleting the original file

7) Profit!

It is possible to do online resize, without stopping the VM. libvirtd supports this natively:

Find the block device name. Should be something like "vda"

$ virsh domblklist <libvirtd_vm_name>

Resize the virtual device:

$ virsh blockresize --domain <libvirtd_vm_name> --path <block_device_name> --size <new_size>

Here is an example were I expand the vda disk from 50GB to 51GB for undercloud VM.

[root@localhost ~]# virsh domblklist undercloud
Target     Source
------------------------------------------------
vda        /home/images/undercloud.qcow2

Now take a look at the .qcow2 image file's details:

[root@localhost ~]# qemu-img info /home/images/undercloud.qcow2
image: /home/images/undercloud.qcow2
file format: qcow2
virtual size: 50G (53687091200 bytes)
disk size: 38G
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

Now let's resize the vda block device:

[root@localhost ~]# virsh blockresize undercloud vda 51G
Block device 'vda' is resized

And confirm:

[root@localhost ~]# qemu-img info /home/images/undercloud.qcow2
image: /home/images/undercloud.qcow2
file format: qcow2
virtual size: 51G (54760833024 bytes)
disk size: 38G
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false
[root@localhost ~]#

Then you can use this script inside the VM to show the commands to resize the block devices and fs: https://github.com/mircea-vutcovici/scripts/blob/master/vol_resize.sh. Here is a sample output:

mvutcovi@ubuntu1904:~$ wget -q https://raw.githubusercontent.com/mircea-vutcovici/scripts/master/vol_resize.sh

mvutcovi@ubuntu1904:~$ df /
Filesystem     1K-blocks    Used Available Use% Mounted on
/dev/vda1       15414216 7928904   6682600  55% /

mvutcovi@ubuntu1904:~$ sudo bash vol_resize.sh --block-device /dev/vda1
sfdisk -d /dev/vda > dev_vda-partition-table-$(date +%F_%H%M%S).txt   # Backup MS-DOS partition table for /dev/vda block device.
parted -s /dev/vda resizepart 1   # Resize MS-DOS partition /dev/vda1
# Update kernel with new partition table from disk
partx -u /dev/vda
partprobe /dev/vda
blockdev --rereadpt /dev/vda
kpartx -u /dev/vda
resize2fs /dev/vda1    # Resize ext3 or ext4 filesystem
mvutcovi@ubuntu1904:~$ 

If you are using LVM within the VM the simplest way to do this woudl be to add a new virtual disk to the VM and expand the volume group and logical volumes onto that.

To check if you are using LVM run sudo pvs; sudo vgs; sudo lvs, you will get something like this out:

PV         VG     Fmt  Attr PSize  PFree
/dev/vda1  vgWWW  lvm2 a-   30.00g    0

VG     #PV #LV #SN Attr   VSize  VFree
vgWWW    1   2   0 wz--n- 30.00g    0

LV   VG    Attr   LSize 
root vgWWW -wi-ao 28.80g
swap vgWWW -wi-ao  1.19g

if the VM's OS is using LVM. In the above example the VM has a 30Gbyte vdisk, configured using LVM with one volume group called vgWWW containing two logical volumes, one for swap and one for everything else.

If LV is in use in the VM:

1. Shut down the VM
2. On the host create a new virtual disk file and add it to the VM
3. Restart the VM and login
4. Mark the new drive as being a physical volume for LVM with sudo pvcreate /dev/vdb
5. Extend the volume group to include this new block of disk with sudo vgextend vgWWW /dev/vdb
6. Extend the logical volume to take up the new space with sudo lvextend --extents +100%FREE /dev/vgWWW/root (or something like sudo lvextend --size +8G /dev/vgWWW/root if you don't want to grow it all the way, this example would add 8Gb to the volume)
7. Resize the filesystem with resize2fs /dev/vgWWW/root

Note: the above assumes the vg/lv names are the same as my example which is unlikely, change as appropriate, also if the VM already had a virtual drive called vdb the new one will be something else (vdc, and so on)

Note: resize2fs will only work on ext2, ext3 and ext4 filesystem. If you are using something else it will error and do nothing.

Note: as you are resizing a live filesystem resize2fs won't prompt you to run fsck first as it would for an unmounted filesystem, it will just go ahead. You might want to run a read-only filesystem check there are no issues before proceeding.

This way you can expand the partition you want:

    # see what partitions you have? 
    virt-filesystems --long -h --all -a olddisk

    truncate -r olddisk newdisk
    truncate -s +5G newdisk

    # Note "/dev/sda2" is a partition inside the "olddisk" file.
    virt-resize --expand /dev/sda2 olddisk newdisk

See more examples here.

There is a possibility to increase the size of your VM's disk without rebooting the VM if you're using virtio drive and LVM.

1. Use virt-manager or virsh to define a new empty disk
2. The kernel should see a new /dev/vdb

3. (Optional) Create a primary partition with fdisk to get /dev/vdb1, then use kpartx -a /dev/vdb to reread the partition table

4. Use vgextend vg_name /dev/vdb1 (or /dev/vdb if you did not create a partition)

5. Use lvextend -l +100%FREE /dev/vg_name/lv_name
6. Use resize2fs /dev/vg_name/lv_name to extend your filesystem

You're done.

Another way to do it

truncate -s +2G vm1.img 

go in the make a disk rescan and after you can do your lvm resize.

If you have LVM in your VM then this is crazy easy and fast.

1. Turn off VM (guest machine).
2. In your host machine add new storage device.
3. Turn on guest.
4. Open System -> Administration -> Logical Volume Management (or type sudo system-config-lvm in terminal)*.

I found the GUI quite intuitive, but follow next steps if you have problems.

1. Open "Uninitialized Entities" and find your new drive.
2. After selecting new drive, below the image you should find an initialize button. Press it.
3. It will ask you about partitioning - you don't need it.
4. When done add your drive to existing volume group.
5. Finally you need to grow your logical volume(s) - this is done in Logical View of the group by editing properties of your volume.
6. Save and your done. No need to restart VM.

Note! At least on CentOS 6 LVM GUI is not installed by default, but you can install it with yum install system-config-lvm.