A High-Availability Cluster for Linux
Whenever the network interfaces need to be brought up or down I have used Red Hat's supplied ifup and ifdown scripts. This makes the network interface configuration more tightly integrated with the GUI network configuration tools. The node configuration files, /etc/cluster.d/nodename.conf, allow you to specify the Ethernet NIC and its purpose on each node in the cluster. My two node configuration files are shown in Listings 1 and 2.
To implement the MAC address take over, one important addition must be made to the Red Hat Ethernet configuration files. You must add a line to the /etc/sysconfig/network-scripts/ifcfg-eth2 file to set the MAC address. eth2 is the redundant interface in my case, so I need it to take over the MAC address of the main service interface on the other node in the cluster. In other words, the MAC address of eth2 on serv2 must be the same as the MAC address of eth0 on serv1. The line 'MACADDR=00:10:4B:63:1C:08' was appended to this file on node serv2. The Red Hat ifup command will use this variable when bringing up an interface. A similar modification must be made to each node.
If you use an Ethernet switch (instead of a hub), it will be necessary to set the MAC address cache timeout to a suitable period to avoid the cluster losing communication with the LAN clients after a MAC address takeover. I set ours to 20 seconds for the ports which are connected directly to the nodes. Consult your switch manual or vendor if you need information on how to do this. It can usually be done via the console cable.
I have created service mirror description files and crontab entries for /etc/hosts, passwd/group files and the entire /etc/clusterd/ directory so that I can administer the cluster from a single node. This greatly simplifies cluster configuration. To avoid confusion, I found it helpful to create a DNS alias for each service used on the cluster which points to the primary node for that service. Thus, when I need to configure Samba, all I need to do is remotely log in to samba.yourdomainname.com. If the secondary node for a service is configured by mistake, any changes will be ignored until the primary node fails.
Currently for my system, only two nodes may be in a cluster. Scaling this up to clusters of more than two nodes should not be difficult, although instead of MAC address takeover, a different approach will probably have to be used because of the large number of NICs required for larger clusters.
Several useful utilities enabled me to do efficient mirroring. rsync is an invaluable utility which uses the rsync algorithm. This program will look for changes in files and mirror only the parts which have changed rather than the whole file. It also checks if the file has been updated by examining the modification date and file size before doing any further comparisons. ssh (secure shell) can also be used between the nodes in conjunction with rsync so that the mirrored data is sent via an encrypted and authenticated connection. Alternatively, you can use rsh if you prefer.
When rsync is doing file comparisons, it uses the file's date and time; therefore, it is vital that the nodes all agree on the same time. I chose to run the netdate utility every hour from cron. The nodes used a list of remote trusted time sources. To make sure a failed node boots with the correct time, the CMOS PC clock is updated after running netdate.
rsync was configured so that files in the source directory which do not exist on the target directory are deleted. This behaviour is necessary to avoid accumulative and excessive disk usage on the target node. If this setting is not used, a user connected to a Samba file share would effectively not be able to delete any file on the mirrored node. The same goes for almost all applications. clusterd is configured to create backups of deleted or changed files when the resynchronization procedure is in progress. This can help minimize the risk of data loss in the event of mirroring failure prior to a node takeover. Subsequent removal of backup files would necessitate some human intervention, after it has been confirmed that files or data were not lost after the node recovery. This is done using the --backup option of rsync version 2.2.1. You may find it more CPU efficient to turn off the rsync algorithm and fully mirror files which have changed instead of mirroring the changes; however, this will utilize more network bandwidth.
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- August 2015 Issue of Linux Journal: Programming