add blog article about linux virtual machines
Signed-off-by: Nico Schottelius <nico@ikn.schottelius.org>
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Nico Schottelius
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blog/linux-virtual-machines-a-real-pain.mdwn
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blog/linux-virtual-machines-a-real-pain.mdwn
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[[!meta title="Linux virtual machine software is a real pain"]]
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This report is a about my todays experience with virtual machines.
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## UML (user-mode-linux)
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It began this morning, when I tried to setup a new virtual
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machine with [User mode Linux](http://user-mode-linux.sourceforge.net/).
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I could easily reuse an existing installation using
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copy-on-write with the following command:
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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
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After I issued
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apt-get update && apt-get dist-upgrade
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in the virtual machine, it hung. It did not react to new ssh connections.
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I've seen this behaviour quite often with **user mode Linux**, when I have
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"a lot" of disk input/output. Ok, I wanted to use some kind of
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framework for my virtual machines anyway, so for the time being,
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let's forget about uml and try the libvirt+kvm.
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## Libvirt
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The [libvirt](http://libvirt.org/) project looks quite promising
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from its documentation, especially in combination with
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[virt-manager](http://virt-manager.org/). Trying to create a
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new virtual machine with virt-manager is kind of strange, because
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it insists of having an installation medium. Though, locating the
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Debian live CD is not so difficult. But then came the big problem:
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When I tried to create a new disk image, virt-manager just hung
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for several minutes, without the host system doing anything.
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Some time before I had massive problems using virt-manager and selecting
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a different pool for the images, which caused several problems when
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trying to start the VM.
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But well, let's give [virsh](http://www.libvirt.org/apps.html) a try,
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the command line utility to manage libvirt. Creating a new disk image
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with virsh is pretty easy:
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vol-create-as default jr.img 8G
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A bit confusing is the fact that the **vol-create** command without
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**-as** prefix expects a XML-file as input. Having a look at the
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other create commands confirms guess:
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ikn:~% LANG=C LC_ALL=C virsh help | grep create
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create create a domain from an XML file
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net-create create a network from an XML file
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nodedev-create create a device defined by an XML file on the node
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pool-create create a pool from an XML file
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pool-create-as create a pool from a set of args
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vol-create create a vol from an XML file
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vol-create-from create a vol, using another volume as input
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vol-create-as create a volume from a set of args
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ikn:~% virsh --version
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0.7.4
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Some commands do not support creation from the command line, but
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only from an XML-file, which makes virsh useless for interactive
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and scripting use.
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This brings me to the new kid on the block: ganeti
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## Ganeti
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When I first experienced problems with libvirt, some people pointed
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me to [ganeti](http://code.google.com/p/ganeti/)
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(to speak truth, it was one of the ganeti developers).
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Until today I delayed this idea, but after the problems with libvirt
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I decided to give ganeti-2.0.5-1 (Debian package) a try. First of all
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I tried to follow the
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[installation tutorial](http://ganeti-doc.googlecode.com/svn/ganeti-1.2/install.html)
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referenced on the homepage, which is heavily orientated on using
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[Xen](http://www.xen.org/) and [LVM](http://tldp.org/HOWTO/LVM-HOWTO/), both
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of them I do not plan to use. Trying to get ganeti running, I was meeting
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some interesting problems:
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[11:26] tee:root# gnt-cluster init ganeti.schottelius.org
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Failure: prerequisites not met for this operation:
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This host's IP resolves to the private range (127.0.1.1). Please fix DNS or /etc/hosts.
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This is described in the ganeti manual and easily fixed by commenting out the
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relevant entry in ***/etc/hosts***:
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[11:27] tee:root# grep tee.schottelius.org /etc/hosts
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#127.0.1.1 tee.schottelius.org tee
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After that I was a bit confused by ganeti not finding its cluster name:
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[11:27] tee:root# gnt-cluster init ganeti.schottelius.org
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Failure: can't resolve hostname 'ganeti.schottelius.org'
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[11:28] tee:root# ping ganeti.schottelius.org
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PING ganeti.schottelius.org (77.109.138.195) 56(84) bytes of data.
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64 bytes from ganeti.schottelius.org (77.109.138.195): icmp_seq=1 ttl=64 time=0.026 ms
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^C
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--- ganeti.schottelius.org ping statistics ---
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1 packets transmitted, 1 received, 0% packet loss, time 0ms
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rtt min/avg/max/mdev = 0.026/0.026/0.026/0.000 ms
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Retrying two times "solved" the problem, which is a bit confusing for me
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as ganeti and ping both use the same resolver library. After that I met the
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"no-lvm-problem":
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[11:38] tee:root# gnt-cluster init -b br0 ganeti.schottelius.org
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Failure: prerequisites not met for this operation:
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Error: volume group 'xenvg' missing
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specify --no-lvm-storage if you are not using lvm
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Specifying the required parameter led me into a new problem:
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[11:38] tee:root# gnt-cluster init -b br0 --no-lvm-storage ganeti.schottelius.org
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Failure: prerequisites not met for this operation:
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Invalid master netdev given (xen-br0): 'Device "xen-br0" does not exist.'
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Which is interesting, **ganeti seems to ignore the bridge paramater -b**.
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So, to use ganeti, I **renamed the bridge from br0 to xen-br0** in
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***/etc/network/interfaces***:
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auto xen-br0
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iface xen-br0 inet manual
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bridge_ports eth1
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And finally I was able to initialise the ganeti cluster:
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[15:06] tee:root# gnt-cluster init -b br0 --no-lvm-storage ganeti.schottelius.org
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Then I tried to join the host into the cluster, which failed, but retrieving
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status information also failed:
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[15:06] tee:root# gnt-node add tee.schottelius.org
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Node tee.schottelius.org already in the cluster (as tee.schottelius.org) - please retry with '--readd'
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[15:07] tee:root# gnt-node list
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Node DTotal DFree MTotal MNode MFree Pinst Sinst
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tee.schottelius.org ? ? ? ? ? 0 0
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Trying to re-add it, results in an error without an error message
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and does not fix the problem:
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[15:07] tee:root# gnt-node add --readd tee.schottelius.org
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The authenticity of host 'tee.schottelius.org (77.109.138.222)' can't be established.
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RSA key fingerprint is c7:d0:a8:32:ad:f0:9b:fa:1e:77:d5:1f:64:d8:9b:db.
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Are you sure you want to continue connecting (yes/no)? yes
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Thu Dec 31 15:08:23 2009 - INFO: Readding a node, the offline/drained flags were reset
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Thu Dec 31 15:08:23 2009 - INFO: Node will be a master candidate
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Failure: command execution error:
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[15:08] tee:root#
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[15:32] tee:root# gnt-node list
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Node DTotal DFree MTotal MNode MFree Pinst Sinst
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tee.schottelius.org ? ? ? ? ? 0 0
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[15:34] tee:root#
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At that point I was pointed to the
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[more recent documentation](http://ganeti-doc.googlecode.com/svn/ganeti-2.0/install.html)
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of ganeti and began from scratch:
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[16:22] tee:vm# gnt-cluster destroy --yes-do-it
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[16:23] tee:vm# gnt-cluster init --no-lvm-storage ganeti.schottelius.org
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[16:26] tee:vm# gnt-node list
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Node DTotal DFree MTotal MNode MFree Pinst Sinst
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tee.schottelius.org ? ? ? ? ? 0 0
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After double checking that the needed daemons are running
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(/etc/init.d/ganeti restart), I got a good hint: One has to specify
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the supervisor to use during initialisation:
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[16:34] tee:vm# gnt-cluster destroy --yes-do-it
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[16:35] tee:vm# gnt-cluster init --no-lvm-storage -t kvm ganeti.schottelius.org
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[16:36] tee:vm# gnt-node list
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Node DTotal DFree MTotal MNode MFree Pinst Sinst
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tee.schottelius.org ? ? 19.6G 2.7G 17.6G 0 0
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Now I tried to add a new virtual machine instance, which resulted in another
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error:
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[16:51] tee:vm# gnt-instance add -t file -s 4G -o debootstrap -n tee.schottelius.org jr.nachtbrand.ch
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Failure: prerequisites not met for this operation:
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Hypervisor parameter validation failed on node tee.schottelius.org: Instance kernel '/boot/vmlinuz-2.6-kvmU' not found or not a file
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This seems to be some kind ganeti logic to have the kernel outside the
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block device, which is similar to the user mode Linux approach. After linking one
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of the host kernels and its initrd adding an instance succeeded:
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[16:59] tee:/boot# ln -s vmlinuz-2.6.30-2-amd64 vmlinuz-2.6-kvmU
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[16:59] tee:/boot# ln -s initrd.img-2.6.30-2-amd64 initrd-2.6-kvmU
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[17:00] tee:vm# gnt-instance add -t file -s 4G -o debootstrap -n tee.schottelius.org jr.nachtbrand.ch
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[17:01] tee:/boot# gnt-instance list
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Instance Hypervisor OS Primary_node Status Memory
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jr.nachtbrand.ch kvm debootstrap tee.schottelius.org running 128M
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It is also correctly connected to the bridge, seen as valid by **gnt-os**
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and **gnt-cluster verify** looks good:
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[17:14] tee:/boot# brctl show
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bridge name bridge id STP enabled interfaces
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br0 8000.000000000000 no
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xen-br0 8000.0015176a26f7 no eth1
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tap4
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[17:16] tee:/boot# gnt-os diagnose
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OS: debootstrap [global status: valid]
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Node: tee.schottelius.org, status: VALID (path: /usr/share/ganeti/os/debootstrap)
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[17:17] tee:/boot# gnt-cluster verify
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Thu Dec 31 17:17:24 2009 * Verifying global settings
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Thu Dec 31 17:17:24 2009 * Gathering data (1 nodes)
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Thu Dec 31 17:17:24 2009 * Verifying node tee.schottelius.org (master)
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Thu Dec 31 17:17:24 2009 * Verifying instance jr.nachtbrand.ch
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Thu Dec 31 17:17:24 2009 * Verifying orphan volumes
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Thu Dec 31 17:17:24 2009 * Verifying remaining instances
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Thu Dec 31 17:17:24 2009 * Verifying N+1 Memory redundancy
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Thu Dec 31 17:17:24 2009 * Other Notes
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Thu Dec 31 17:17:24 2009 - NOTICE: 1 non-redundant instance(s) found.
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Thu Dec 31 17:17:24 2009 * Hooks Results
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As specified in the documentation, I tried to connect to the console:
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[17:28] tee:/boot# gnt-instance console jr.nachtbrand.ch
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[17:30] tee:/boot# gnt-instance console --show-cmd jr.nachtbrand.ch
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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'
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The problem is that the newly debootstrapped system
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*does not have a serial console setup*.
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As you can see, in the evening of this day I had a lot of new experiences,
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but *no reliable running virtualisation framework*. That brings me to the
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end of this report:
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* User mode Linux does not work reliable under some I/O load.
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* Virt-manager is absolutely not able to change the simplest parameters.
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* Virsh is unusable, if you don't want to edit XML-files.
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* Ganeti has a lot of unhandled problems and still relies very much on Xen + LVM.
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As next Monday my vacation ends, I will have a look at the commercial virtualisation
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frameworks. For the folks of the named FOSS stuff above: Guys, you've to improve
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a lot, until one can call your software "good and clean software".
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[[!tag unix vm]]
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3
tags/vm.mdwn
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3
tags/vm.mdwn
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Virtual machine related stuff
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[[!inline pages="tagged(vm)" archive="yes" show=0 quick=no]]
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