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KVM – detach disk

sudo virsh detach-disk --domain test --persistent --live --target vdb

KVM – Add / Attach Disk

Create disk:
qemu-img create \
-f qcow2 \
-o lazy_refcounts=on,preallocation=falloc \
$FILEPATH \
[size]G

qemu-img comes with various options for setting the allocation when creating new disk images.

preallocation=metadata – allocates the space required by the metadata but doesn’t allocate any space for the data. This is the quickest to provision but the slowest for guest writes.

preallocation=falloc – allocates space for the metadata and data but marks the blocks as unallocated. This will provision slower than metadata but quicker than full. Guest write performance will be much quicker than metadata and similar to full.

preallocation=full – allocates space for the metadata and data and will therefore consume all the physical space that you allocate (not sparse). All empty allocated space will be set as a zero. This is the slowest to provision and will give similar guest write performance to falloc.

Convert disk:
mv disk.qcow2 disk.qcow2.bak
qemu-img convert -O qcow2 -o lazy_refcounts=on,preallocation=falloc disk.qcow2.bak disk.qcow2


Attach disk:
virsh attach-disk [vm ID] \
--source /path/to/disk.qcow2 \
--target vd[x] \
--persistent \
--subdriver qcow2

Enable Virsh Console Access For KVM Guests

To do so, log in to your guest machine via SSH or Virt-manager or Cockpit and run the following commands to enable and start a serial console:

systemctl enable serial-getty@ttyS0.service
systemctl start serial-getty@ttyS0.service

connect to console from Host
virsh console

EFI boot entry

show
efibootmgr
root@hphost:~# efibootmgr
BootCurrent: 0002
Timeout: 0 seconds
BootOrder: 0000,0004,0002,0001
Boot0000* debian
Boot0001* ubuntu
Boot0002* rEFInd Boot Manager
Boot0004* Windows Boot Manager

change order
sudo efibootmgr -o 0004,0002,0001,0000

change the EFI boot manager timeout
efibootmgr --timeout=4

delete entry
efibootmgr -Bb 0003

KVM – set CPU resources

virsh vcpucount <vm_name>
virsh setvcpus <vm_name> <max-number-of-CPUs> –maximum –config
virsh setvcpus <vm_name> <number-of-CPUs> –config
virsh setvcpus <vm_name> <number-of-CPUs> –live

KVM add interface live and permanent

virsh net-list --all

 Name State Autostart Persistent
----------------------------------------------------
default inactive yes yes
inner-network active yes yes
outer-network active yes yes


virsh attach-interface --domain debian11 --type bridge --source inner --mac 52:54:00:29:5c:2e --model virtio --config --live

KVM – create new virtual network

Create new bridge

sudo ip link add <bridge-name> type bridge

Add interface to bridge

sudo ip link set <ethX> up
sudo ip link set <ethX> master <bridge-name>

Add IP address to bridge and bring it up

sudo ip address add dev <bridge-name> 192.168.0.90/24

sudo ip link set dev <bridge-name> up

Create file bridged-network.xml

<network>
    <name>bridged-network</name>
    <forward mode="bridge" />
    <bridge name="<bridge-name>" />
</network>

Add bridged-network to our KVM

sudo virsh net-define bridged-network.xml

Activate network and set autostart

sudo virsh net-start bridged-network 
sudo virsh net-autostart bridged-network

Verify

virsh net-list --all

KVM – Migrate VM to another host

1. copy the VM’s disks from /var/lib/libvirt/images on src host to the same dir on destination host
2. on the source host run virsh dumpxml VMNAME > domxml.xml and copy this xml to the destination host
3. on the destination host run virsh define domxml.xml
4. start the VM.

• If the disk location differs, you need to edit the xml’s devices/disk node to point to the image on the destination host
• If the VM is attached to custom defined networks, you’ll need to either edit them out of the xml on the destination host or redefine them as well
  1. On source machine virsh net-dumpxml NETNAME > netxml.xml
  2. copy netxml.xml to target machine
  3. On target machine virsh net-define netxml.xml && virsh net-start NETNAME & virsh net-autostart NETNAME

resize img size for QEMU / KVM

STOP VM !

Add +5G do disk

qemu-img resize chr-7.2.img +5G

Change disk size to 20G

qemu-img resize chr-7.2.img 20G

This example will convert a raw image file named image.img to a qcow2 image file.

qemu-img convert -f raw -O qcow2 image.img image.qcow2

Rescan resized partition

partprobe /dev/vdb

or

growpart /dev/vdb 1

Resize partition

resize2fs /dev/vdb1

Useful commands

virsh destroy vm_name
virsh list
virsh edit vm_name

Set CPU resources

virsh vcpucount <vm_name>
virsh setvcpus <vm_name> <max-number-of-CPUs> --maximum --config
virsh setvcpus <vm_name> <number-of-CPUs> --config
virsh setvcpus <vm_name> <number-of-CPUs> --live

Mount partition and chroot in rescue mode

Run linux from Linux Live (debian.ubuntu, slackware tec.)

Determine what is your main partition

Example output:

create mount point and mount root partition

Mount the necessary file system directories by running the following commands:

Mount /usr and /var partition if necessary

Chroot to md0

chroot /mnt/md0