Data Service Use of Single or Multiple Logical Hosts

In Sun Cluster, there can be any number of logical hosts, so your data service implementation should not depend on a certain quantity. You must decide whether your data service will keep its data in just one or in multiple logical hosts.

Generally, it is easier to design and implement a data service that uses just one logical host. In that case, all of the data service's data is placed only on that logical host's diskset. The data service needs just one set of daemons. A physical host runs the daemons for that data service only if the physical host currently masters the single logical host that the data service uses. When the physical host takes over mastery of the logical host, the data service's start method can start up the daemons. When the physical host is giving up mastery of the logical host, the data service's stop method can stop the daemons. In some cases, killing the daemons by sending a kill signal will suffice.

If you use multiple logical hosts, you must be able to split the data service's data into disjoint sets. The sets must be split so that no operation the data service needs to perform requires data from more than one set.

Consider Sun's HA-NFS product, which has multiple file systems with different data residing in each file system. For HA-NFS, each logical host has its own set of NFS^(TM) file systems. Each physical host NFS shares the file systems that belong to the logical hosts that it masters. The sets of NFS file systems belonging to the two logical hosts are disjoint.

Using multiple logical hosts enables some rudimentary load balancing: when both physical hosts are up, each physical host can master one of the logical hosts and handles the data service's traffic for that logical host. Thus, both physical hosts are doing useful work in addition to acting as standbys for each other.

For some data services, splitting the data into disjoint collections such that no data service operation requires more than one collection is not feasible. The in.named example described in Chapter 2 "Sample Data Service", is such a data service. It has only one set of interdependent data files, and it would be difficult to split them into disjoint sets.

Configure the data service to use just one of the logical hosts, unless the data is easily split into disjoint collections and there is significant benefit to the rudimentary load balancing enabled by use of multiple logical hosts.