Solaris 10 10/08 Installation Guide: Planning for Installation and Upgrade

State Database Replicas Guidelines and Requirements

You should distribute state database replicas across slices, drives, and controllers, to avoid single points of failure. You want a majority of replicas to survive a single component failure. If you lose a replica, when a device fails, for example, the failure might cause problems with running Solaris Volume Manager software or when rebooting the system. Solaris Volume Manager software requires at least half of the replicas to be available to run, but a majority (half plus one) to reboot into multiuser mode.

For detailed instructions about creating and administering state database replicas, see Solaris Volume Manager Administration Guide.

Selecting Slices for State Database Replicas

Before selecting slices for state database replicas, consider the following guidelines and recommendations.

Task	Description
Choose a dedicated slice	You should create state database replicas on a dedicated slice of at least 4 MB per replica. If necessary, you could create state database replicas on a slice that is to be used as part of a RAID-0 or RAID-1 volume. You must create the replicas before you add the slice to the volume.
Resize a slice	By default, the size of a state database replica is 4 MB or 8192 disk blocks. Because your disk slices might not be that small, you can resize a slice to hold the state database replica. For information about resizing a slice, see Chapter 11, Administering Disks (Tasks), in System Administration Guide: Devices and File Systems.
Choose a slice that is not in use	You can create state database replicas on slices that are not in use. The part of a slice that is reserved for the state database replica should not be used for any other purpose.
	You cannot create state database replicas on existing file systems, or the root (`/`), `/usr`, and `swap` file systems. If necessary, you can create a new slice (provided a slice name is available) by allocating space from `swap` and then put state database replicas on that new slice.
Choosing a slice that becomes a volume	When a state database replica is placed on a slice that becomes part of a volume, the capacity of the volume is reduced by the space that is occupied by the replica or replicas. The space that is used by a replica is rounded up to the next cylinder boundary and this space is skipped by the volume.

Choosing the Number of State Database Replicas

Before choosing the number of state database replicas, consider the following guidelines.

A minimum of 3 state database replicas are recommended, up to a maximum of 50 replicas per Solaris Volume Manager disk set. The following guidelines are recommended:
- For a system with only a single drive: put all three replicas in one slice.
- For a system with two to four drives: put two replicas on each drive.
- For a system with five or more drives: put one replica on each drive.
Additional state database replicas can improve the mirror's performance. Generally, you need to add two replicas for each mirror you add to the system.
If you have a RAID-1 volume that is to be used for small-sized random I/O (for example, for a database), consider your number of replicas. For best performance, ensure that you have at least two extra replicas per RAID-1 volume on slices (and preferably on disks and controllers) that are unconnected to the RAID-1 volume.

Distributing State Database Replicas Across Controllers

If multiple controllers exist, replicas should be distributed as evenly as possible across all controllers. This strategy provides redundancy if a controller fails and also helps balance the load. If multiple disks exist on a controller, at least two of the disks on each controller should store a replica.