Mirroring Guidelines

This section provides guidelines for planning the mirroring of your cluster configuration.

Mirroring Multihost Disks

Mirroring all multihost disks in a Sun Cluster configuration enables the configuration to tolerate single-disk failures. Sun Cluster software requires that you mirror all multihost disks across disk expansion units.

Consider the following points when mirroring multihost disks.

• Separate disk expansion units - Each submirror of a given mirror or plex should reside in a different multihost disk expansion unit.

• Disk space - Mirroring doubles the amount of necessary disk space.

• Three-way mirroring - Solstice DiskSuite software and VERITAS Volume Manager (VxVM) support three-way mirroring. However, Sun Cluster requires only two-way mirroring.

• Number of metadevices - Under Solstice DiskSuite software, mirrors consist of other metadevices such as concatenations or stripes. Large configurations might contain a large number of metadevices. For example, seven metadevices are created for each logging UFS file system.

• Differing disk sizes - If you mirror to a disk of a different size, your mirror capacity is limited to the size of the smallest submirror or plex.

For more information about multihost disks, see Sun Cluster 3.0 U1 Concepts.

Mirroring the Root Disk

Add this planning information to the "Local File System Layout Worksheet" in the Sun Cluster 3.0 U1 Release Notes.

For maximum availability, you should mirror root (/), /usr, /var, /opt, and swap on the local disks. Under VxVM, you encapsulate the root disk and mirror the generated subdisks. However, mirroring the root disk is not a requirement of Sun Cluster.

Before deciding whether to mirror the root disk, consider the risks, complexity, cost, and service time for the various alternatives concerning the root disk. There is no single mirroring strategy that works for all configurations. You might want to consider your local Enterprise Services representative's preferred solution when deciding whether to mirror root.

See your volume manager documentation and "Installing and Configuring Solstice DiskSuite Software" or "Installing and Configuring VxVM Software" for instructions on mirroring the root disk.

Consider the following issues and guidelines when deciding whether to mirror the root disk.

• Complexity - Mirroring the root disk adds complexity to system administration and complicates booting in single-user mode.

• Backups - Regardless of whether or not you mirror the root disk, you also should perform regular backups of root. Mirroring alone does not protect against administrative errors. Only a backup plan enables you to restore files that have been accidentally altered or deleted.

• Quorum devices - Do not use a disk configured as a quorum device to mirror a root disk.

• Quorum - Under Solstice DiskSuite software, in failure scenarios in which metadevice state database quorum is lost, you cannot reboot the system until maintenance is performed. See the Solstice DiskSuite documentation for information about the metadevice state database and state database replicas.

• Separate controllers - Highest availability includes mirroring the root disk on a separate controller.

• Boot disk - You can set up the mirror to be a bootable root disk so that you can boot from the mirror if the primary boot disk fails.

• Secondary root disk - With a mirrored root disk, the primary root disk can fail but work can continue on the secondary (mirror) root disk. At a later point, the primary root disk might return to service (perhaps after a power cycle or transient I/O errors) and subsequent boots are performed by using the primary root disk specified in the OpenBoot^TM PROM boot-device field. In this situation no manual repair task occurs, but the drive starts working well enough to boot. Note that a Solstice DiskSuite resync does occur. A resync requires a manual step when the drive is returned to service.

  If changes were made to any files on the secondary (mirror) root disk, they would not be reflected on the primary root disk during boot time (causing a stale submirror). For example, changes to the /etc/system file would be lost. Some Solstice DiskSuite administrative commands might have changed the /etc/system file while the primary root disk was out of service.

  The boot program does not check whether it is booting from a mirror or an underlying physical device, and the mirroring becomes active partway through the boot process (after the metadevices are loaded). Before this point, the system is vulnerable to stale submirror problems.