====== Libvirt tips and tricks ======


===== libvirt machine =====

Display available machine types:

<code bash>
$ qemu-system-x86_64 -machine help | grep -v deprecated
Supported machines are:
microvm              microvm (i386)
pc-i440fx-9.2        Standard PC (i440FX + PIIX, 1996)
pc-i440fx-9.1        Standard PC (i440FX + PIIX, 1996)
pc-i440fx-9.0        Standard PC (i440FX + PIIX, 1996)
pc-i440fx-8.2        Standard PC (i440FX + PIIX, 1996)
pc-i440fx-8.1        Standard PC (i440FX + PIIX, 1996)
pc-i440fx-8.0        Standard PC (i440FX + PIIX, 1996)
pc-i440fx-7.2        Standard PC (i440FX + PIIX, 1996)
pc-i440fx-7.1        Standard PC (i440FX + PIIX, 1996)
pc                   Standard PC (i440FX + PIIX, 1996) (alias of pc-i440fx-10.0)
pc-i440fx-10.0       Standard PC (i440FX + PIIX, 1996) (default)
pc-q35-9.2           Standard PC (Q35 + ICH9, 2009)
pc-q35-9.1           Standard PC (Q35 + ICH9, 2009)
pc-q35-9.0           Standard PC (Q35 + ICH9, 2009)
pc-q35-8.2           Standard PC (Q35 + ICH9, 2009)
pc-q35-8.1           Standard PC (Q35 + ICH9, 2009)
pc-q35-8.0           Standard PC (Q35 + ICH9, 2009)
pc-q35-7.2           Standard PC (Q35 + ICH9, 2009)
pc-q35-7.1           Standard PC (Q35 + ICH9, 2009)
q35                  Standard PC (Q35 + ICH9, 2009) (alias of pc-q35-10.0)
pc-q35-10.0          Standard PC (Q35 + ICH9, 2009)
isapc                ISA-only PC
none                 empty machine
x-remote             Experimental remote machine
</code>

Basically **pc-i440fx** is old hardware (no UFI boot etc) for old software and **pc-q35** is new hardware what you probably need. Then version number is a frozen state of the machine specs, use the latest one except if you need to migrate the VM between different versions of qemu, in this case use highest version supported by all version of qemu you have.

Choosing an alias **q35** here might looks a good idea but VM will implicitly use new version when you upgrade qemu and can can break things. So rather upgrade machine manually after OS major upgrade.


To view current machine type of a VM:

<code bash>
$ virsh dumpxml <vm> | grep machine=
    <type arch='x86_64' machine='pc-q35-3.1'>hvm</type>
</code>

To edit:

<code bash>
$ virsh edit <vm>
# then look for machine= and set chosen version (i.e. "pc-q35-10.0") and restart
# the VM using virsh shutdown <vm> then virsh start <vm> (reboot wont work)
</code>

===== SSD/NVMe fstrim =====


View current disk definition in libvirt:

<code bash>
$ virsh dumpxml <vm> | grep -A2 '<driver'
      <driver name='qemu' type='raw' cache='none' io='native'/>
      <source dev='/dev/mapper/vg0-vm' index='1'/>
      <backingStore/>
</code>


You need **discard='unmap' detect_zeroes='unmap'** in **<driver />** so discard will propagate up to the host.

<code bash>
$ virsh edit <vm>
# look for <driver and add discard='unmap' detect_zeroes='unmap' and restart the VM
# using virsh shutdown <vm> and virsh start <vm> (reboot wont work)
</code>


Then in VM run:

<code bash>
$ fstrim -av
</code>

This will be done automatically by **fstrim.timer** service which should be enabled by default on recent debian. If not:

<code bash>
$ systemctl enable --now fstrim.timer
Created symlink '/etc/systemd/system/timers.target.wants/fstrim.timer' → '/usr/lib/systemd/system/fstrim.timer'.
</code>