In Conclusion


To build a highly available server pair that can be easily administered, you need to be able to copy configuration files from the primary server to the backup server when you make changes to files on the primary server. One way to copy configuration files from the primary server to the backup server in a highly available server pair is to use the rsync utility in conjunction with the Open Secure Shell transport.

It is tempting to think that this method of copying files between two servers can be used to create a very low-cost highly available data solution using only locally attached disk drives on the primary and the backup servers. However, if the data stored on the disk drives attached to the primary server changes frequently, as would be the case if the highly available server pair is an NFS server or an SQL database server, then using rsync combined with Open SSH is not an adequate method of providing highly available data. The data on the primary server could change, and the disk drive attached to the primary server could crash, long before an rsync cron job has the chance to copy the data to the backup server.[11] When your highly available server pair needs to modify data frequently, you should do one of the following things to make your data highly available:

  • Install a distributed filesystem such as Red Hat's Global File System (GFS) on both the primary and the backup server (see Appendix D for a partial list of additional distributed filesystems).

  • Connect both the primary and the backup server to a highly available SAN and then make the filesystem where data is stored a resource under Heartbeat's control.

  • Attach both the primary and the backup server to a single shared SCSI bus that uses RAID drives to store data and then make the filesystem where data is stored a resource under Heartbeat's control.

When you install the Heartbeat package on the primary and backup server in a highly available server pair and make the shared storage file system (as described in the second two bullets) where your data is stored a resource under Heartbeat's control, the filesystem will only be mounted on one server at a time (under normal circumstances, the primary server). When the primary server crashes, the backup server mounts the shared storage filesystem where the data is stored and launches the SQL or NFS server to provide cluster nodes with access to the data. Using a highly available server pair and shared storage in this manner ensures that no data is lost as a result of a failover.

I'll describe how to make a resource highly available using the Heartbeat package later in this part of the book, but before I do, I want to build on the concepts introduced in this chapter and describe how to clone a system using the SystemImager package.

[11]Using rsync to make snapshots of your data may, however, be an acceptable disaster recovery solution.



The Linux Enterprise Cluster. Build a Highly Available Cluster with Commodity Hardware and Free Software
Linux Enterprise Cluster: Build a Highly Available Cluster with Commodity Hardware and Free Software
ISBN: 1593270364
EAN: 2147483647
Year: 2003
Pages: 219
Authors: Karl Kopper

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