document some kvm stuff and lvm resizing

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 [[ganeti]] on top and most documentation

here should not be necessary in day-to-day Ganeti operations.

[KVM switch]: https://en.wikipedia.org/wiki/KVM_switch

[Kernel-based Virtual Machine]: https://en.wikipedia.org/wiki/Kernel-based_Virtual_Machine

Tutorials

=========

Rebooting

---------

Rebooting should be done with a specific procedure, documented in [[upgrades]].

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:

[QCOW2]: https://en.wikipedia.org/wiki/Qcow

    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.

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 29GB:

    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 29GB is in use. You can also see there's a layer of LVM

volumes inside the guest, so the actual allocation is for 40GB:

    root@vineale:/srv# pvs

      PV         VG         Fmt  Attr PSize  PFree

      /dev/sdb   vg_vineale lvm2 a--  40,00g 5,00g

That 40GB size is allocated inside the `QCOW` image.

    lvconvert --type cache-pool --cachemode writethrough --poolmetadata "$vg"/srv-cache-meta "$vg"/srv-cache

    lvconvert --type cache --cachepool "$vg"/srv-cache "$vg"/srv

# Resizing

Assume we want to grow this partition to take the available free space

in the PV:

    root@vineale:/srv# lvs

      LV   VG         Attr       LSize  Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert

      srv  vg_vineale -wi-ao---- 35,00g                                                    

    root@vineale:/srv# pvs

      PV         VG         Fmt  Attr PSize  PFree

      /dev/sdb   vg_vineale lvm2 a--  40,00g 5,00g

    root@vineale:~# pvdisplay 

      --- Physical volume ---

      PV Name               /dev/sdb

      VG Name               vg_vineale

      PV Size               40,00 GiB / not usable 4,00 MiB

      Allocatable           yes 

      PE Size               4,00 MiB

      Total PE              10239

      Free PE               1279

      Allocated PE          8960

      PV UUID               CXKO15-Wze1-xY6y-rOO6-Tfzj-cDSs-V41mwe

To resize the partition to take up all available free space, you

should do the following:

 1. stop services and processes using the partition (will obviously vary):

        service apache2 stop

 2. unmount the filesystem:

        umount /srv

 3. check the filesystem:

        fsck -y -f /dev/mapper/vg_vineale-srv

 4. extend the filesystem using the extent notation to take up all

    available space:

        lvextend vg_vineale/srv -l +1279

 5. grow the filesystem:

        resize2fs -p /dev/mapper/vg_vineale-srv

 6. recheck the filesystem:

        fsck  -f -y /dev/mapper/vg_vineale-srv

 7. remount the filesystem and start processes:

        mount /srv

        service apache2 start

Note: this assumes there is free space on the physical volume. If

there isn't you will need to add disks to the volume group, and grow

the physical volume. For example:

    pvcreate /dev/md123

    vgextend vg_vineale /dev/md123

If the underlying disk was grown, which happens in virtual hosting

environments, you can also just extend the physical volume:

    pvextend /dev/sdb

See also the [upstream documentation][].

[upstream documentation]: http://www.tldp.org/HOWTO/LVM-HOWTO/extendlv.html