Veritas Volume Manager

Veritas Volume Manager is an unbundled software package that can be purchased separately via Sun, or direct from Veritas, and does not come as part of the standard Solaris 10 release. This product has traditionally been used for managing Sun's larger StorEdge disk arrays. It is widely used for performing Virtual Volume Management functions on large scale systems such as Sun, Sequent, and HP. Although Veritas Volume Manager also provides the capability to mirror the OS drive, in actual industry practice, you'll still see SVM used to mirror the OS drive, even on large Sun servers that use Veritas Volume Manager to manage the remaining data. It used to be much more robust than the older Solstice DiskSuite productthe predecessor to the Solaris Volume Manager, providing tools that identify and analyze storage access patterns so that I/O loads can be balanced across complex disk configurations. SVM is now a much more robust product but the difference is negligible.

Veritas Volume Manager is a complex product that would take much more than this chapter to describe in detail. This chapter will, however, introduce you to the Veritas Volume Manager and some of the terms you will find useful.

The Volume Manager builds virtual devices called volumes on top of physical disks. A physical disk is the underlying storage device (media), which may or may not be under Volume Manager control. A physical disk can be accessed using a device name such as /dev/rdsk/c#t#d. The physical disk, as explained in Chapter 1, can be divided into one or more slices.

Volumes are accessed by the Solaris file system, a database, or other applications in the same way physical disk partitions would be accessed. Volumes and their virtual components are referred to as Volume Manager objects.

There are several Volume Manager objects that the Volume Manager uses to perform disk management tasks (see Table 10.9).

Table 10.9. Volume Manager Objects

Object Name	Description
VM Disk	A contiguous area of disk space from which the Volume Manager allocates storage. Each VM disk corresponds to at least one partition. A VM disk usually refers to a physical disk in the array.
Disk Group	A collection of VM disks that share a common configuration. The default disk group used to be rootdg (the root disk group) in versions prior to version 4, but there is now no default disk group assigned. Additional disk groups can be created, as necessary. Volumes are created within a disk group; a given volume must be configured from disks belonging to the same disk group. Disk groups allow the administrator to group disks into logical collections for administrative convenience.
Subdisk	A set of contiguous disk blocks; subdisks are the basic units in which the Volume Manager allocates disk space. A VM disk can be divided into one or more subdisks.
Plex	Often referred to as mirrors; a plex consists of one or more subdisks located on one or more disks, forming one side of a mirror configuration. The use of two or more plexes forms a functional mirror.
Volume	A virtual disk device that appears to be a physical disk partition to applications, databases, and file systems, but does not have the physical limitations of a physical disk partition. Volumes are created within a disk group; a given volume must be configured from disks belonging to the same disk group.

Volume Manager objects can be manipulated in a variety of ways to optimize performance, provide redundancy of data, and perform backups or other administrative tasks on one or more physical disks without interrupting applications. As a result, data availability and disk subsystem throughput are improved.

Veritas Volume Manager manages disk space by using contiguous sectors. The application formats the disks into only two slices: Slice 3 and Slice 4. Slice 3 is called a private area and Slice 4 is the public area. Slice 3 maintains information about the virtual to physical device mappings, while Slice 4 provides space to build the virtual devices. The advantage to this approach is that there is almost no limit to the number of subdisks you can create on a single drive. In a standard Solaris disk partitioning environment, there is an eight-partition limit per disk.

The names of the block devices for virtual volumes created using Veritas Volume Manager are found in the /dev/vx/dsk/<disk_group>/<volume_name> directory, and the names of the raw devices are found in the /dev/vx/rdsk/<disk_group>/<volume_name> directory. The following is an example of a block and raw logical device name:

/dev/vx/dsk/apps/vol01 - block device /dev/vx/rdsk/apps/vol01 - \raw device