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Some other additional information can be found in the documentation of the Thomas-Krenn.AG cluster (The author of this howto is working in the cluster development there, that is the reason why he was able to write this howto :-). The full documentation with interesting illustrations is currently only available in German:
http://my.thomas-krenn.com/service_support/index.php/page.242
This article shows how to setup a OpenVZ high availability (HA) cluster using the data replication software DRBD and the cluster manager Heartbeat. In this example the two machines builing building the cluster run on CentOS 4.3. The article also shows how to do kernel updates in the cluster, including necessary steps like recompiling of new DRBD userspace tools. For this purpose, kernel 2.6.8-022stab078.10 (containing DRBD module 0.7.17) is used as initial kernel version, and kernel 2.6.8-022stab078.14 (containing DRBD module 0.7.20) as updated kernel version.
<b>Update:</b> this howto currently does not describe details on OpenVZ Kernel 2.6.18, which contains DRBD version 8.*. Meanwhile, some hints on using OpenVZ Kernel 2.6.18 with DRBD 8 can be found in [http://forum.openvz.org/index.php?t=msg&th=3213&start=0& this thread in the forum]. Additional information about clustering of virtual machines can be found in the following paper: [http://www.linuxtag.org/2006/fileadmin/linuxtag/dvd/12080-paper.pdf(PDF, 145K)] Some other additional information can be found in the documentation of the Thomas-Krenn.AG cluster (The author of this howto is working in the cluster development there, that is the reason why he was able to write this howto :-). The full documentation with interesting illustrations is currently only [http://www.thomas-krenn.com/en/service-support/knowledge-center/cluster/documentation.html available in German]: An excellent presentation and overview by Werner Fischer, Thomas-Krenn.AG is available here http://www.profoss.eu/index.php/main/content/download/355/3864/file/werner-fischer.pdf.
== Prerequisites ==
The OpenVZ kernel already includes the DRBD module. The DRBD userspace tools and the cluster manager Heartbeat must be provided seperately. As the API version of the DRBD userspace tools must exactly match the API version of the module, compile them yourself. Also compile Heartbeat yourself, as at the time of this writing the CentOS extras repository only contained an old CVS version of Heartbeat.
On a hardware node for production use there should not be any applications application that are is not really needed for running OpenVZ (any things which are not needed by OpenVZ should run in a VE for security reasons). As a result, compile DRBD and Heartbeat on another machine running CentOS 4.3 (in this example I used a virtual machine on a VMware Server).
=== Compiling Heartbeat ===
! other install options
| no firewall, no SELinux
| no firewall, no SELinux|-! package groups| deactivated everything, only kept vim-enhanced| deactivated everything, only kept SELinuxt vim-enhanced
|}
Get the OpenVZ kernel and utilities and install them on both nodes, as described in [[quick installation]]. Update grub configuration to use the OpenVZ kernel by default. Disable starting of OpenVZ on system boot on both nodes (OpenVZ will be started and stopped by Heartbeat):
<pre>
[root@ovz-node1 ~]# chkconfig --del vzoff
[root@ovz-node1 ~]#
</pre>
== Setting up DRBD ==
'''On each of the two nodes create a partition that acts as underlying DRBD device. ''' The partitions should have exectly exactly the same size (I created a 10 GB partition hda3 using fdisk on each node for this example). Note that it might be necessary to reboot the machines to re-read the partition table.
Install the rpm of the DRBD userspace tools on both nodes:
[root@ovz-node1 ~]# mkdir -p /vz/cluster/etc/sysconfig
[root@ovz-node1 ~]# mkdir -p /vz/cluster/var
[root@ovz-node1 ~]# cp -a /etc/vz .orig /vz/cluster/etc/vz/[root@ovz-node1 ~]# cp -a /etc/sysconfig/vz-scripts .orig /vz/cluster/etc/sysconfig/vz-scripts[root@ovz-node1 ~]# cp -a /var/vzquota .orig /vz/cluster/var/vzquota
[root@ovz-node1 ~]# umount /dev/drbd0
[root@ovz-node1 ~]#
[root@ovz-node1 ~]#
</pre>
Create the Heartbeat configuration file ha.cf and copy it to <code>/etc/ha.d/ha.cf </code> on both nodes. Details about this file can be found at http://www.linux-ha.org/ha.cf. Below is an example configuration which uses the two network connections and also a serial connection for heartbeat packets:
<pre>
# Heartbeat logging configuration
respawn hacluster /usr/lib64/heartbeat/ipfail
</pre>
Create the Heartbeat configuration file authkeys and copy it to <code>/etc/ha.d/authkeys </code> on both nodes. Set the permissions of this file to 600. Details about this file can be found at http://www.linux-ha.org/authkeys. Below is an example:
<pre>
auth 1
1 sha1 PutYourSuperSecretKeyHere
</pre>
Create the Heartbeat configuration file haresources and copy it to <code>/etc/ha.d/haresources </code> on both nodes. Details about this file can be found at http://www.linux-ha.org/haresources. Note that it is not necessary to configure IPs for gratuitous arp here. The gratuitous arp is done by OpenVZ itself, through <code>/etc/sysconfig/network-scripts/ifup-venet</code> and <code>/usr/lib/vzctl/scripts/vps-functions</code>. Below is an examplefor the haresources file:
<pre>
ovz-node1 datadiskdrbddisk::r0 Filesystem::/dev/drbd0::/vz::ext3 vz MailTo::youremail@yourdomain.tld
</pre>
Finally, you can now start heartbeat on both nodes:
[root@ovz-node1 ~]#
</pre>
== Before going in production: testing, testing, testing, and ...hm... testing! ==
The installation of the cluster is finished at this point. Before putting the cluster in production it is very important to test the cluster. Because of all the possible different kinds of hardware that you may have, you may encounter problems when a failover is necessary. And as the cluster is about high availability, such problems must be found before the cluster is used for production.
Here is one example: The e1000 driver that is included in kernels < 2.6.12 has a problem when a cable gets unplugged while broadcast packets are still being sent out on that interface. When using broadcast communication in Heartbeat on a crossover link, this fills up the transmit ring buffer on the adapter (the buffer is full after about 8 minutes after the cable got unplugged). Using unicast communication in Heartbeat fixes the problem for example. Details see: http://www.osdl.org/developer_bugzilla/show_bug.cgi?id=699#c22
Without testing you may not be aware of such problems and may face them when the cluster is in production and a failover would be necessary. So test your cluster carefully!
Possible tests can include:
* power outage test of active node
* power outage test of passive node
* network connection outage test of eth0 of active node
* network connection outage test of eth0 of passive node
* network connection outage test of crossover network connection
* ...
As mentioned above, some problems only arise after an outage lasts longer than some minutes. So do the tests also with a duration of >1h for example.
Before you start to test, build a test plan. Some valueable information on that can be found in chapter 3 "Testing a highly available Tivoli Storage Manager cluster environment" of the Redbook ''IBM Tivoli Storage Manager in a Clustered Environment'', see http://www.redbooks.ibm.com/abstracts/sg246679.html. In this chapter it is mentioned that the experience of the authoring team is that the testing phase must be at least two times the total implementation time for the cluster.
== Before installing kernel updates: testing again ==
New OpenVZ kernel often include driver updates. This kernel for examples includes an update of the e1000 module: http://openvz.org/news/updates/kernel-022stab078.21
To avoid to overlook problems with new components (such as a newer kernel), it is necessary to re-do the tests mentioned above. But as the cluster is already in production, a second cluster (test cluster) with the same hardware as the main cluster is needed. Use this test cluster to test updates of the kernel or main OS updates for the hardware node before putting them on the production cluster.
I know this is not an easy task, as it is time-consuming and needs additional hardware only for testing. But when really business-critical applications are running on the cluster, it is very good to now that the cluster works fine also with new updates installed on the hardware node. In many cases a dedicated test cluster and the time efford for the testing of updates may cause too much costs. If you cannot do such test of updates, keep in mind that over time (when you must install security updates of the OS or the kernel) you have a cluster that you have not tested in this configuration.
If you need a tested cluster (also with tested kernel updates), you may take a look on this Virtuozzo cluster: http://www.thomas-krenn.com/cluster
== How to do OpenVZ kernel updates when it contains a new DRBD version ==
As mentioned above, it is important to use the correct version of the DRBD userspace tools. When an OpenVZ kernel contains a new DRBD version, it is important that the DRBD API version of the userspace tools matches the API version of the DRBD module that is included in the OpenVZ kernel. The API versions can be found at http://svn.drbd.org/drbd/branches/drbd-0.7/ChangeLog. The best way is to always use the version of the DRBD userspace tools that matches the version of the DRBD module that is included in the OpenVZ kernel.
Ensure after every update of OpenVZ tools that OpenVZ is not started on system boot. To disable starting of OpenVZ on system boot execute on both nodes:
<pre>
[root@ovz-node1 ~]# chkconfig --del vzoff
[root@ovz-node1 ~]#
</pre>
== Live-Switchover with the help of checkpointing ==
With the help of [[Checkpointing_and_live_migration|checkpointing]] it is possible to do live switchovers.
<b>Important:</b> although this HOWTO currently describes the use of DRBD 0.7, it is necessary to use DRBD 8 to be able to use this live-switchover feature reliable. Some hints on using OpenVZ Kernel 2.6.18 with DRBD 8 can be found in [http://forum.openvz.org/index.php?t=msg&th=3213&start=0& this thread in the forum].
The following scripts are written by Thomas Kappelmueller. They should be placed at /root/live-switchover/ on both nodes. To activate the scripts execute the following commands on both nodes:
<pre>
[root@ovz-node1 ~]# ln -s /root/live-switchover/openvz /etc/init.d/
[root@ovz-node1 ~]# ln -s /root/live-switchover/live_switchover.sh /root/bin/
[root@ovz-node1 ~]#
</pre>
It is also necessary to replace <code>vz</code> by an adjusted initscript (<code>openvz</code> in this example). So /etc/ha.d/haresources has the following content on both nodes:
<pre>
ovz-node1 drbddisk::r0 Filesystem::/dev/drbd0::/vz::ext3 openvz MailTo::youremail@yourdomain.tld
</pre>
=== Script cluster_freeze.sh ===
<pre>
#!/bin/bash
#Script by Thomas Kappelmueller
#Version 1.0
LIVESWITCH_PATH='/vz/cluster/liveswitch'
if [ -f $LIVESWITCH_PATH ]
then
rm -f $LIVESWITCH_PATH
fi
RUNNING_VE=$(vzlist -1)
for I in $RUNNING_VE
do
BOOTLINE=$(cat /etc/sysconfig/vz-scripts/$I.conf | grep -i "^onboot")
if [ $I != 1 -a "$BOOTLINE" = "ONBOOT=\"yes\"" ]
then
vzctl chkpnt $I
if [ $? -eq 0 ]
then
vzctl set $I --onboot no --save
echo $I >> $LIVESWITCH_PATH
fi
fi
done
exit 0
</pre>
=== Script cluster_unfreeze.sh ===
<pre>
#!/bin/bash
#Script by Thomas Kappelmueller
#Version 1.0
LIVESWITCH_PATH='/vz/cluster/liveswitch'
if [ -f $LIVESWITCH_PATH ]
then
FROZEN_VE=$(cat $LIVESWITCH_PATH)
else
exit 1
fi
for I in $FROZEN_VE
do
vzctl restore $I
if [ $? != 0 ]
then
vzctl start $I
fi
vzctl set $I --onboot yes --save
done
rm -f $LIVESWITCH_PATH
exit 0
</pre>
=== Script live_switchover.sh ===
<pre>
#!/bin/bash
#Script by Thomas Kappelmueller
#Version 1.0
ps -eaf | grep 'vzctl enter' | grep -v 'grep' > /dev/null
if [ $? -eq 0 ]
then
echo 'vzctl enter is active. please finish before live switchover.'
exit 1
fi
ps -eaf | grep 'vzctl exec' | grep -v 'grep' > /dev/null
if [ $? -eq 0 ]
then
echo 'vzctl exec is active. please finish before live switchover.'
exit 1
fi
echo "Freezing VEs..."
/root/live-switchover/cluster_freeze.sh
echo "Starting Switchover..."
/usr/lib64/heartbeat/hb_standby
</pre>
=== Script openvz ===
<pre>
#!/bin/bash
#
# openvz Startup script for OpenVZ
#
start() {
/etc/init.d/vz start > /dev/null 2>&1
RETVAL=$?
/root/live-switchover/cluster_unfreeze.sh
return $RETVAL
}
stop() {
/etc/init.d/vz stop > /dev/null 2>&1
RETVAL=$?
return $RETVAL
}
status() {
/etc/init.d/vz status > /dev/null 2>&1
RETVAL=$?
return $RETVAL
}
# See how we were called.
case "$1" in
start)
start
;;
stop)
stop
;;
status)
status
;;
*)
echo $"Usage: openvz {start|stop|status}"
exit 1
esac
exit $RETVAL
</pre>
[[Category: HOWTO]]
