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add blog article about linux virtual machines

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Signed-off-by: Nico Schottelius <nico@ikn.schottelius.org>
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Nico Schottelius 2010-01-01 22:47:24 +01:00
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[[!meta title="Linux virtual machine software is a real pain"]]
This report is a about my todays experience with virtual machines.
## UML (user-mode-linux)
It began this morning, when I tried to setup a new virtual
machine with [User mode Linux](http://user-mode-linux.sourceforge.net/).
I could easily reuse an existing installation using
copy-on-write with the following command:
linux umid=vm4 uml_dir=/home/nico/vm/uml con1=pts ubda=/home/nico/vm/cow/vm4,/home/nico/vm/images/debian eth0=tuntap,,,192.168.4.1 mem=4096M
After I issued
apt-get update && apt-get dist-upgrade
in the virtual machine, it hung. It did not react to new ssh connections.
I've seen this behaviour quite often with **user mode Linux**, when I have
"a lot" of disk input/output. Ok, I wanted to use some kind of
framework for my virtual machines anyway, so for the time being,
let's forget about uml and try the libvirt+kvm.
## Libvirt
The [libvirt](http://libvirt.org/) project looks quite promising
from its documentation, especially in combination with
[virt-manager](http://virt-manager.org/). Trying to create a
new virtual machine with virt-manager is kind of strange, because
it insists of having an installation medium. Though, locating the
Debian live CD is not so difficult. But then came the big problem:
When I tried to create a new disk image, virt-manager just hung
for several minutes, without the host system doing anything.
Some time before I had massive problems using virt-manager and selecting
a different pool for the images, which caused several problems when
trying to start the VM.
But well, let's give [virsh](http://www.libvirt.org/apps.html) a try,
the command line utility to manage libvirt. Creating a new disk image
with virsh is pretty easy:
vol-create-as default jr.img 8G
A bit confusing is the fact that the **vol-create** command without
**-as** prefix expects a XML-file as input. Having a look at the
other create commands confirms guess:
ikn:~% LANG=C LC_ALL=C virsh help | grep create
create create a domain from an XML file
net-create create a network from an XML file
nodedev-create create a device defined by an XML file on the node
pool-create create a pool from an XML file
pool-create-as create a pool from a set of args
vol-create create a vol from an XML file
vol-create-from create a vol, using another volume as input
vol-create-as create a volume from a set of args
ikn:~% virsh --version
0.7.4
Some commands do not support creation from the command line, but
only from an XML-file, which makes virsh useless for interactive
and scripting use.
This brings me to the new kid on the block: ganeti
## Ganeti
When I first experienced problems with libvirt, some people pointed
me to [ganeti](http://code.google.com/p/ganeti/)
(to speak truth, it was one of the ganeti developers).
Until today I delayed this idea, but after the problems with libvirt
I decided to give ganeti-2.0.5-1 (Debian package) a try. First of all
I tried to follow the
[installation tutorial](http://ganeti-doc.googlecode.com/svn/ganeti-1.2/install.html)
referenced on the homepage, which is heavily orientated on using
[Xen](http://www.xen.org/) and [LVM](http://tldp.org/HOWTO/LVM-HOWTO/), both
of them I do not plan to use. Trying to get ganeti running, I was meeting
some interesting problems:
[11:26] tee:root# gnt-cluster init ganeti.schottelius.org
Failure: prerequisites not met for this operation:
This host's IP resolves to the private range (127.0.1.1). Please fix DNS or /etc/hosts.
This is described in the ganeti manual and easily fixed by commenting out the
relevant entry in ***/etc/hosts***:
[11:27] tee:root# grep tee.schottelius.org /etc/hosts
#127.0.1.1 tee.schottelius.org tee
After that I was a bit confused by ganeti not finding its cluster name:
[11:27] tee:root# gnt-cluster init ganeti.schottelius.org
Failure: can't resolve hostname 'ganeti.schottelius.org'
[11:28] tee:root# ping ganeti.schottelius.org
PING ganeti.schottelius.org (77.109.138.195) 56(84) bytes of data.
64 bytes from ganeti.schottelius.org (77.109.138.195): icmp_seq=1 ttl=64 time=0.026 ms
^C
--- ganeti.schottelius.org ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.026/0.026/0.026/0.000 ms
Retrying two times "solved" the problem, which is a bit confusing for me
as ganeti and ping both use the same resolver library. After that I met the
"no-lvm-problem":
[11:38] tee:root# gnt-cluster init -b br0 ganeti.schottelius.org
Failure: prerequisites not met for this operation:
Error: volume group 'xenvg' missing
specify --no-lvm-storage if you are not using lvm
Specifying the required parameter led me into a new problem:
[11:38] tee:root# gnt-cluster init -b br0 --no-lvm-storage ganeti.schottelius.org
Failure: prerequisites not met for this operation:
Invalid master netdev given (xen-br0): 'Device "xen-br0" does not exist.'
Which is interesting, **ganeti seems to ignore the bridge paramater -b**.
So, to use ganeti, I **renamed the bridge from br0 to xen-br0** in
***/etc/network/interfaces***:
auto xen-br0
iface xen-br0 inet manual
bridge_ports eth1
And finally I was able to initialise the ganeti cluster:
[15:06] tee:root# gnt-cluster init -b br0 --no-lvm-storage ganeti.schottelius.org
Then I tried to join the host into the cluster, which failed, but retrieving
status information also failed:
[15:06] tee:root# gnt-node add tee.schottelius.org
Node tee.schottelius.org already in the cluster (as tee.schottelius.org) - please retry with '--readd'
[15:07] tee:root# gnt-node list
Node DTotal DFree MTotal MNode MFree Pinst Sinst
tee.schottelius.org ? ? ? ? ? 0 0
Trying to re-add it, results in an error without an error message
and does not fix the problem:
[15:07] tee:root# gnt-node add --readd tee.schottelius.org
The authenticity of host 'tee.schottelius.org (77.109.138.222)' can't be established.
RSA key fingerprint is c7:d0:a8:32:ad:f0:9b:fa:1e:77:d5:1f:64:d8:9b:db.
Are you sure you want to continue connecting (yes/no)? yes
Thu Dec 31 15:08:23 2009 - INFO: Readding a node, the offline/drained flags were reset
Thu Dec 31 15:08:23 2009 - INFO: Node will be a master candidate
Failure: command execution error:
[15:08] tee:root#
[15:32] tee:root# gnt-node list
Node DTotal DFree MTotal MNode MFree Pinst Sinst
tee.schottelius.org ? ? ? ? ? 0 0
[15:34] tee:root#
At that point I was pointed to the
[more recent documentation](http://ganeti-doc.googlecode.com/svn/ganeti-2.0/install.html)
of ganeti and began from scratch:
[16:22] tee:vm# gnt-cluster destroy --yes-do-it
[16:23] tee:vm# gnt-cluster init --no-lvm-storage ganeti.schottelius.org
[16:26] tee:vm# gnt-node list
Node DTotal DFree MTotal MNode MFree Pinst Sinst
tee.schottelius.org ? ? ? ? ? 0 0
After double checking that the needed daemons are running
(/etc/init.d/ganeti restart), I got a good hint: One has to specify
the supervisor to use during initialisation:
[16:34] tee:vm# gnt-cluster destroy --yes-do-it
[16:35] tee:vm# gnt-cluster init --no-lvm-storage -t kvm ganeti.schottelius.org
[16:36] tee:vm# gnt-node list
Node DTotal DFree MTotal MNode MFree Pinst Sinst
tee.schottelius.org ? ? 19.6G 2.7G 17.6G 0 0
Now I tried to add a new virtual machine instance, which resulted in another
error:
[16:51] tee:vm# gnt-instance add -t file -s 4G -o debootstrap -n tee.schottelius.org jr.nachtbrand.ch
Failure: prerequisites not met for this operation:
Hypervisor parameter validation failed on node tee.schottelius.org: Instance kernel '/boot/vmlinuz-2.6-kvmU' not found or not a file
This seems to be some kind ganeti logic to have the kernel outside the
block device, which is similar to the user mode Linux approach. After linking one
of the host kernels and its initrd adding an instance succeeded:
[16:59] tee:/boot# ln -s vmlinuz-2.6.30-2-amd64 vmlinuz-2.6-kvmU
[16:59] tee:/boot# ln -s initrd.img-2.6.30-2-amd64 initrd-2.6-kvmU
[17:00] tee:vm# gnt-instance add -t file -s 4G -o debootstrap -n tee.schottelius.org jr.nachtbrand.ch
[17:01] tee:/boot# gnt-instance list
Instance Hypervisor OS Primary_node Status Memory
jr.nachtbrand.ch kvm debootstrap tee.schottelius.org running 128M
It is also correctly connected to the bridge, seen as valid by **gnt-os**
and **gnt-cluster verify** looks good:
[17:14] tee:/boot# brctl show
bridge name bridge id STP enabled interfaces
br0 8000.000000000000 no
xen-br0 8000.0015176a26f7 no eth1
tap4
[17:16] tee:/boot# gnt-os diagnose
OS: debootstrap [global status: valid]
Node: tee.schottelius.org, status: VALID (path: /usr/share/ganeti/os/debootstrap)
[17:17] tee:/boot# gnt-cluster verify
Thu Dec 31 17:17:24 2009 * Verifying global settings
Thu Dec 31 17:17:24 2009 * Gathering data (1 nodes)
Thu Dec 31 17:17:24 2009 * Verifying node tee.schottelius.org (master)
Thu Dec 31 17:17:24 2009 * Verifying instance jr.nachtbrand.ch
Thu Dec 31 17:17:24 2009 * Verifying orphan volumes
Thu Dec 31 17:17:24 2009 * Verifying remaining instances
Thu Dec 31 17:17:24 2009 * Verifying N+1 Memory redundancy
Thu Dec 31 17:17:24 2009 * Other Notes
Thu Dec 31 17:17:24 2009 - NOTICE: 1 non-redundant instance(s) found.
Thu Dec 31 17:17:24 2009 * Hooks Results
As specified in the documentation, I tried to connect to the console:
[17:28] tee:/boot# gnt-instance console jr.nachtbrand.ch
[17:30] tee:/boot# gnt-instance console --show-cmd jr.nachtbrand.ch
ssh -q -oEscapeChar=none -oHashKnownHosts=no -oGlobalKnownHostsFile=/var/lib/ganeti/known_hosts -oUserKnownHostsFile=/dev/null -oHostKeyAlias=ganeti.schottelius.org -oBatchMode=yes -oStrictHostKeyChecking=yes -t root@tee.schottelius.org '/usr/bin/socat STDIO,echo=0,icanon=0 UNIX-CONNECT:/var/run/ganeti/kvm-hypervisor/ctrl/jr.nachtbrand.ch.serial'
The problem is that the newly debootstrapped system
*does not have a serial console setup*.
As you can see, in the evening of this day I had a lot of new experiences,
but *no reliable running virtualisation framework*. That brings me to the
end of this report:
* User mode Linux does not work reliable under some I/O load.
* Virt-manager is absolutely not able to change the simplest parameters.
* Virsh is unusable, if you don't want to edit XML-files.
* Ganeti has a lot of unhandled problems and still relies very much on Xen + LVM.
As next Monday my vacation ends, I will have a look at the commercial virtualisation
frameworks. For the folks of the named FOSS stuff above: Guys, you've to improve
a lot, until one can call your software "good and clean software".
[[!tag unix vm]]

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Virtual machine related stuff
[[!inline pages="tagged(vm)" archive="yes" show=0 quick=no]]