Planning Your Logical Host Configuration

A disk group stores the data for one or more data services. Generally, several data services share a logical host, and therefore fail over together. If you want to enable a particular data service to fail over independently of all other data services, then assign a logical host to that data service alone, and do not allow any other data services to share it.

As part of the installation and configuration, you need to establish the following for each logical host:

• Default master - Each logical host can potentially be mastered by any physical host to which it is connected.

• HA administrative file system - This is a mount point on the logical host for the administrative file system. Refer to "Planning Your File System Layout on the Multihost Disks", for more information.

• vfstab file name - Each logical host needs a separate vfstab file to store file system information. This name is generally vfstab.logicalhost.

• Disk group or Diskset - Each disk group or diskset has its own name. By convention, the disk group or diskset name and the logical host name are the same, but you can give the disk group or diskset another name.

Use the logical host worksheet in "Configuration Worksheets", to record this information.

Consider these points when planning your logical host configuration:

• If a data service does not put a heavy load on the CPU or memory, then you will not gain a load-balancing advantage by switching it over separately.

• Use care when load balancing an N+1 configuration. If the data service puts a heavy load on the CPU or memory, then you should limit the workload on the hot-standby node. A large workload on the hot-standby node compromises its ability to take over should any active node fail.

• If the data service software is relatively reliable and starts up quickly, then you will not gain much availability by failing it over separately.

• If the data service uses only a small amount of disk space, you might waste a lot of disk space by putting it on a separate logical host, because you must have at least a mirrored pair of drives per disk group.

• The administrative burden increases as the number of logical hosts grows.