2.2.7 Determining Your Multihost Disk Requirements

Unless you are using a RAID5 configuration, all multihost disks must be mirrored in Sun Cluster configurations. This enables the configuration to tolerate single-disk failures. Refer to "2.5.1 Mirroring Guidelines", and to your volume management documentation, for more information.

Determine the amount of data that you want to move to the Sun Cluster configuration. If you are not using RAID5, double that amount to allow disk space for mirroring. With RAID5, you need extra space equal to 1/(# of devices -1). Use the worksheets in Appendix A, Configuration Worksheets and Examples, to help plan your disk requirements.

Consider these points when planning your disk requirements:

• Sun Cluster supports several multihost disk expansion units. Consider the size of disks available with each disk expansion unit when you calculate the amount of data to migrate to Sun Cluster.

• With Sun StorEdge A3000 units, you need only one disk expansion unit because each unit has two controllers. With Sun StorEdge MultiPacks, you must have at least two disk expansion units.

• Under some circumstances, there might be an advantage to merging several smaller file systems into a single larger file system. This reduces the number of file systems to administer and might help speed up cluster takeovers.

• The size of the dump media (backup system) might influence the size of the file systems in your configuration.

• With Solstice DiskSuite, if you have only two disk expansion units, then you must configure dual-string mediators. If you have more than two disk expansion units, you need not configure mediators. Sun StorEdge Volume Manager and Cluster Volume Manager do not support mediators. See the chapter on using dual-string mediators in the Sun Cluster 2.2 System Administration Guide for details on the dual-string mediator feature.

• Refer to the Sun Cluster 2.2 Release Notes for the list of supported disk types.

2.2.7.1 Disk Space Growth

Consider these points when planning for disk space growth:

• Less administration time is required to configure disks during initial configuration than to add them while the system is in service.

• Leaving empty slots in the multihost disk expansion units during initial configuration allows you to add disks easily later.

• When your site needs additional disk expansion units, you might have to reconfigure your data to prevent mirroring within a single disk expansion unit. Therefore, if all the existing disk expansion units are full, the easiest way to add disk expansion units without reorganizing data is to add them in pairs.

2.2.7.2 Size and Number of Disk Drives

Several sizes of disks are supported in multihost disk expansion units. Consider these points when deciding which size drives to use:

• If you use lower capacity drives, you can have more spindles; this increases potential I/O bandwidth, assuming the disks have the same I/O rates.

• If you use higher capacity disks, then fewer devices are required in the configuration. This can help speed up takeovers because takeover time can be partially dependent on the number of drives being taken over.

• You can determine the number of disks needed by dividing the total disk capacity that you have selected (including mirrors) by the disk size in your disk expansion units.