To mirror all multihost disks in a Sun Cluster configuration enables the configuration to tolerate single-device failures. Sun Cluster software requires that you mirror all multihost disks across expansion units. You do not need to use software mirroring if the storage device provides hardware RAID as well as redundant paths to devices.
Separate disk expansion units – Each submirror of a given mirror or plex should reside in a different multihost expansion unit.
Disk space – Mirroring doubles the amount of necessary disk space.
Three-way mirroring – Solstice DiskSuite or Solaris Volume Manager software and VERITAS Volume Manager (VxVM) software support three-way mirroring. However, Sun Cluster software requires only two-way mirroring.
Number of metadevices or volumes – Under Solstice DiskSuite or Solaris Volume Manager software, mirrors consist of other Solstice DiskSuite metadevices or Solaris Volume Manager volumes such as concatenations or stripes. Large configurations might contain a large number of metadevices or volumes.
For more information about multihost disks, see “Multihost Disk Storage” in Sun Cluster Overview for Solaris OS and Sun Cluster Concepts Guide for Solaris OS.
Add this planning information to the Local File System Layout Worksheet.
For maximum availability, mirror root (/), /usr, /var, /opt, and swap on the local disks. Under VxVM, you encapsulate the root disk and mirror the generated subdisks. However, Sun Cluster software does not require that you mirror the root disk.
Before you decide whether to mirror the root disk, consider the risks, complexity, cost, and service time for the various alternatives that concern the root disk. No single mirroring strategy works for all configurations. You might want to consider your local Sun service representative's preferred solution when you decide whether to mirror root.
See your volume-manager documentation and Installing and Configuring Solstice DiskSuite or Solaris Volume Manager Software or SPARC: Installing and Configuring VxVM Software for instructions on how to mirror the root disk.
Consider the following points when you decide whether to mirror the root disk.
Boot disk – You can set up the mirror to be a bootable root disk. You can then boot from the mirror if the primary boot disk fails.
Complexity – To mirror the root disk adds complexity to system administration. To mirror the root disk also complicates booting in single-user mode.
Backups – Regardless of whether 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 – Under Solstice DiskSuite or Solaris Volume Manager software, in failure scenarios in which state database quorum is lost, you cannot reboot the system until maintenance is performed. See your Solstice DiskSuite or Solaris Volume Manager documentation for information about the state database and state database replicas.
Separate controllers – Highest availability includes mirroring the root disk on a separate controller.
Secondary root disk – With a mirrored root disk, the primary root disk can fail but work can continue on the secondary (mirror) root disk. Later, the primary root disk might return to service, for example, after a power cycle or transient I/O errors. Subsequent boots are then performed by using the primary root disk that is specified for the eeprom(1M) boot-device parameter. In this situation, no manual repair task occurs, but the drive starts working well enough to boot. With Solstice DiskSuite or Solaris Volume Manager software, a 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. This condition would cause a stale submirror. For example, changes to the /etc/system file would be lost. With Solstice DiskSuite or Solaris Volume Manager software, some 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 the system is booting from a mirror or from an underlying physical device. The mirroring becomes active partway through the boot process, after the metadevices or volumes are loaded. Before this point, the system is therefore vulnerable to stale submirror problems.