Creating RAID-1 Volumes (Mirrors) During Installation (Overview)

This section discusses the advantages of creating mirrored file systems. The section also describes the Solaris Volume Manager components that are required to create mirrored file systems.

This chapter describes the following topics.

• Why Mirror?

• How Mirroring Works

• Overview of Mirror Components

• Sample Layout for Mirrored File Systems

For additional information about how to create mirrored file systems with Solaris Live Upgrade, see General Guidelines for Creating Mirrored File Systems.

For additional information about how to create mirrored file systems with the custom JumpStart installation method, see filesys Profile Keyword (Creating Mirrored File Systems) and metadb Profile Keyword (Creating State Database Replicas).

Why Mirror?

During the installation or upgrade, you can create mirrored file systems to duplicate your system data over multiple physical disks. By duplicating your data over separate disks, you can protect your data from disk corruption or a disk failure.

The Solaris custom JumpStart and Solaris Live Upgrade installation methods use the Solaris Volume Manager technology to create a mirrored file system. Solaris Volume Manager provides a powerful way to reliably manage your disks and data by using volumes. Solaris Volume Manager enables concatenations, stripes, and other complex configurations. The custom JumpStart and Solaris Live Upgrade installation methods enable a subset of these tasks, such as creating a RAID-1 volume for the root (/) file system. You can create mirrored file systems during your installation or upgrade, eliminating the need to create the mirrored file system after the installation.

Note - The custom JumpStart and Solaris Live Upgrade installation methods only support the creation of RAID-0 and RAID-1 volumes. Other Solaris Volume Manager components, such as RAID-5 volumes, are not supported.

The custom JumpStart installation method supports the creation of mirrored file systems during an initial installation only. Solaris Live Upgrade supports the creation of mirrored file systems during an upgrade.

For detailed information about Solaris Volume Manager software and components, see Solaris Volume Manager Administration Guide.

How Mirroring Works

Solaris Volume Manager uses virtual disks to manage physical disks and their associated data. In Solaris Volume Manager, a virtual disk is called a volume. A volume is a name for a group of physical slices that appear to the system as a single, logical device. Volumes are actually pseudo, or virtual, devices in standard UNIX® terms.

A volume is functionally identical to a physical disk in the view of an application or a file system (such as UFS). Solaris Volume Manager converts I/O requests directed at a volume into I/O requests to the underlying member disks.

Solaris Volume Manager volumes are built from slices (disk partitions) or from other Solaris Volume Manager volumes.

You use volumes to increase performance and data availability. In some instances, volumes can also increase I/O performance. Functionally, volumes behave the same way as slices. Because volumes look like slices, they are transparent to end users, applications, and file systems. Like physical devices, you can use Solaris Volume Manager software to access volumes through block or raw device names. The volume name changes, depending on whether the block or raw device is used.

The custom JumpStart installation method and Solaris Live Upgrade support the use of block devices to create mirrored file systems. See Volume Name Requirements and Guidelines for details about volume names.

When you create a mirrored file system, you create RAID-0 volumes (single-slice concatenations) and RAID-1 volumes (mirrors.) Solaris Volume Manager duplicates data on the concatentations, (submirrors), and treats the submirrors as one mirror volume.

Figure 10-1 shows a mirror that duplicates the root (/) file system over two physical disks.

Figure 10-1 Mirroring the Root File System on Two Disks

Figure 10-1 shows a system with the following configuration.

• The root file system (/) on hdisk0 is included in the single-slice concatenation that is named d31.

• A single-slice concatenation that is named d32 is created on the hard disk that is named hdisk1.

• The mirror that is named d30 consists of the submirrors that are named d31 and d32.

• The mirror duplicates the data in the root file system on both submirrors.

Overview of Mirror Components

The custom JumpStart installation method and Solaris Live Upgrade enable you to create the following components that are required to mirror a file system.

• State database and state database replicas (metadbs)

• Single-slice concatenations (submirrors)

• RAID-1 volumes (mirrors)

This section briefly describes each of these components. For complete information about these components, see Solaris Volume Manager Administration Guide.

State Database and State Database Replicas

The state database is a database that stores information on a physical disk about the state of your Solaris Volume Manager configuration. The state database records and tracks changes that are made to your configuration. Solaris Volume Manager automatically updates the state database when a configuration or state change occurs. Creating a new volume is an example of a configuration change. A submirror failure is an example of a state change.

The state database is actually a collection of multiple, replicated database copies. Each copy, referred to as a state database replica, ensures that the data in the database is always valid. Having copies of the state database protects against data loss from single points of failure. The state database tracks the location and status of all known state database replicas.

Solaris Volume Manager cannot operate until you have created the state database and its state database replicas. A Solaris Volume Manager configuration must have an operating state database.

When you set up your configuration, you can locate the state database replicas on either of the following:

• Dedicated slices

• (Solaris Live Upgrade only) Slices that will later become part of volumes

You can keep more than one copy of a state database on one slice. However, you might make the system more vulnerable to a single point of failure by placing state database replicas on a single slice.

The state database replicas ensure that the data in the state database is always valid. When the state database is updated, each state database replica is also updated. The updates occur one at a time to protect against corruption of all updates if the system crashes.

If your system loses a state database replica, Solaris Volume Manager must identify which state database replicas still contain valid data. Solaris Volume Manager determines this information by using a majority consensus algorithm. This algorithm requires that a majority (half + 1) of the state database replicas be available and in agreement before any of them are considered valid. Because of this majority consensus algorithm, you must create at least three state database replicas when you set up your disk configuration. A consensus can be reached if at least two of the three state database replicas are available.

Each state database replica occupies 4 Mbytes (8192 disk sectors) of disk storage by default. Replicas can be stored on the following devices:

• A dedicated local disk slice

• (Solaris Live Upgrade only) A local slice that will be part of a volume

• (Solaris Live Upgrade only) A local slice that will be part of a UFS logging device

Replicas cannot be stored on the root (/), swap, or /usr slices, or on slices that contain existing file systems or data. After the replicas have been stored, volumes or file systems can be placed on the same slice.

For planning information about state database and state database replica requirements, see State Database Replicas Guidelines and Requirements.

For detailed information about the state database and state database replicas, see Solaris Volume Manager Administration Guide.