KVM is Linux's Kernel-based Virtual Machine (not to be confused with a KVM switch. It's the backing mechanism for our virtualization technologies. This page documents the internals of KVM and the configuration on some of our older nodes. Newer machines should be provisioned with service/ganeti on top and most documentation here should not be necessary in day-to-day Ganeti operations.

RETIRED

This document has been retired since the direct use of KVM was replaced with Ganeti. Ganeti is still using KVM under the hood so the contents here could still be useful.

This documentation is kept for historical reference.

Tutorial

Rebooting

Rebooting should be done with a specific procedure, documented in reboots.

Resizing disks

To resize a disk, you need to resize the QCOW2 image in the parent host.

Before you do this, however, you might also have some wiggle room inside the guest itself, inside the LVM physical volume, see the output of pvs and the LVM cheat sheet.

Once you are sure you need to resize the partition on the host, you need to use the qemu-img command to do the resize.

For example, this will resize (grow!) the image to 50GB, assuming it was smaller before:

qemu-img resize /srv/vmstore/vineale.torproject.org/vineale.torproject.org-lvm 50G

TODO: do we need to stop the host before this? how about repartitionning?

To shrink an image, you need to use the --shrink option but, be careful: the underlying partitions and filesystems need to first be resized otherwise you will have data loss.

Note that this only resizes the disk as seen from the VM. The VM itself might have some partitioning on top of that, and you might need to do filesystem resizes underneath there, including LVM if that's setup there as well. See LVM for details. An example of such a "worst case scenario" occurred in ticket #32644 which has the explicit commands ran on the guest and host for an "LVM in LVM" scenario.

Design

Disk allocation

Disks are allocated on a need-to basis on the KVM host, in the /srv/vmstore. Each disk is a file on the host filesystem, and underneath the guest can create its own partitions. Here is, for example, vineale's disk which is currently taking 29GiB:

root@vineale:/srv# df -h /srv
Sys. de fichiers           Taille Utilisé Dispo Uti% Monté sur
/dev/mapper/vg_vineale-srv    35G     29G  4,4G  87% /srv

On the parent host, it looks like this:

root@macrum:~# du -h /srv/vmstore/vineale.torproject.org/vineale.torproject.org-lvm
29G	/srv/vmstore/vineale.torproject.org/vineale.torproject.org-lvm

ie. only 29GiB is in use. You can also see there's a layer of LVM volumes inside the guest, so the actual allocation is for 40GiB:

root@vineale:/srv# pvs
  PV         VG         Fmt  Attr PSize  PFree
  /dev/sdb   vg_vineale lvm2 a--  40,00g 5,00g

That 40GiB size is allocated inside the QCOW image:

root@macrum:~# file /srv/vmstore/vineale.torproject.org/vineale.torproject.org-lvm
/srv/vmstore/vineale.torproject.org/vineale.torproject.org-lvm: QEMU QCOW Image (v3), 42949672960 bytes