Setup:

You have a host server which has a PV volume and lots of different LV volumes used by different qemu-kvm VMs.

You want to expand the disk of test1.1.1.1.1 VM (called a domain in virsh speak). First find the VM via

~# virsh list

then the disk used by the VM:

~# virsh domblkinfo --domain test1.1.1.1.1 --all --human
Target     Capacity        Allocation      Physical
-----------------------------------------------------
vda        40.000 GiB    (parted) select /dev/vda

You can see that the Physical and Allocation column are almost the same.

Check if you have available space under Free PE / Size in output below:

~# vgdisplay
  --- Volume group ---
  VG Name               centos
  System ID
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  94
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                26
  Open LV               20
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               <1.82 TiB
  PE Size               4.00 MiB
  Total PE              476774
  Alloc PE / Size       331078 / 1.26 TiB
  Free  PE / Size       145696 / 569.12 GiB
  VG UUID               wWFiiE-v4CP-pha0-aAtf-zFiZ-0j6f-RUwUID

Check that the LV volume is part of the VG volume (centos here) by running the lvdisplay and finding the matching LV. Run the lsblk -f as well and you should see the name of LV mount.

Now expand the disk size

~# lvextend -L +100G /dev/mapper/test1.1.1.1.1

The above is the new disk size in GB.

Restart the VM to pick up the new disk size.

Inside of VM run these commands (Cent OS 7 example):

Check which filesystem you want to expand:

[root@quad ~]# lsblk -f
NAME                      FSTYPE      LABEL UUID                                   MOUNTPOINT
sr0
vda
├─vda1                    xfs               32d11a94-18ec-42f0-95fd-ca95fb435ea0   /boot
└─vda2                    LVM2_member       OcgeR3-hMty-TsOV-hkAL-4maE-HcjB-KZNpeY
  ├─centos-lv_root        xfs               02c6e4df-8573-4b36-af74-5e67ba36dbcc   /
  ├─centos-lv_swap        swap              be38e77e-69ab-49c3-88a7-e1dad5459076   [SWAP]
  └─centos-lv_varlibmysql xfs               4b574fe6-830e-45a4-9e1f-894fd3296447   /var/lib/mysql

Here we want to expand vda2, centos-lv_root FS. Run these commands:

sudo yum install -y cloud-utils-growpart
sudo growpart /dev/vda 2
sudo pvresize /dev/vda2
sudo lvextend -l +100%FREE /dev/centos/lv_root
sudo xfs_growfs /

The partition should be expanded now.

If you want to check that the new disk size is seen by the VM use the parted tool:

Before expanding disk:

(parted) select /dev/vda
Using /dev/vda
(parted) print free
Model: Virtio Block Device (virtblk)
Disk /dev/vda: 49.4GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:

Number  Start   End     Size    Type     File system  Flags
        32.3kB  1049kB  1016kB           Free Space
 1      1049kB  1050MB  1049MB  primary  xfs          boot
 2      1050MB  49.4GB  48.3GB  primary               lvm

After expanding the disk and before expanding the partition and filesystem:

(parted) select /dev/vda
Using /dev/vda
(parted) print
Model: Virtio Block Device (virtblk)
Disk /dev/vda: 64.4GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:

Number  Start   End     Size    Type     File system  Flags
 1      1049kB  1050MB  1049MB  primary  xfs          boot
 2      1050MB  49.4GB  48.3GB  primary               lvm

(parted) print free
Model: Virtio Block Device (virtblk)
Disk /dev/vda: 64.4GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:

Number  Start   End     Size    Type     File system  Flags
        32.3kB  1049kB  1016kB           Free Space
 1      1049kB  1050MB  1049MB  primary  xfs          boot
 2      1050MB  49.4GB  48.3GB  primary               lvm
        49.4GB  64.4GB  15.0GB           Free Space

(parted) quit

• CentOS 7 VM host and guest
• qemu-kvm

• Create LVM on Debian 7

• https://stackoverflow.com/questions/12313384/how-to-view-unallocated-free-space-on-a-hard-disk-through-terminal