test

Solaris Volume Manager Administration Guide

Overview of Volumes

A volume is a group of physical slices that appears to the system as a single, logical device. Volumes are actually pseudo, or virtual, devices in standard UNIX® terms.

Note –

Historically, the Solstice DiskSuiteTM product referred to these logical devices as metadevices. However, for simplicity and standardization, this book refers to these devices as volumes.

Classes of Volumes

You create a volume as a RAID-0 (concatenation or stripe) volume, a RAID-1 (mirror) volume, a RAID-5 volume, or a soft partition.

You can use either the Enhanced Storage tool within the Solaris Management Console or the command-line utilities to create and administer volumes.

The following table summarizes the classes of volumes.

Table 3–2 Classes of Volumes

Volume 

Description 

RAID-0 (stripe or concatenation)

Can be used directly, or as the basic building block for mirrors. RAID-0 volumes do not directly provide data redundancy.  

RAID-1 (mirror)

Replicates data by maintaining multiple copies. A RAID-1 volume is composed of one or more RAID-0 volumes that are called submirrors. 

RAID-5

Replicates data by using parity information. In the case of disk failure, the missing data can be regenerated by using available data and the parity information. A RAID-5 volume is generally composed of slices. One slice's worth of space is allocated to parity information, but the parity is distributed across all slices in the RAID-5 volume. 

Soft partition 

Divides a slice or logical volume into one or more smaller, extensible volumes. 

How Volumes Are Used

You use volumes to increase storage capacity, 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, the volumes are transparent to end users, applications, and file systems. As with physical devices, volumes are accessed through block or raw device names. The volume name changes, depending on whether the block or raw device is used. See Volume Names for details about volume names.

You can use most file system commands, including mkfs, mount, umount, ufsdump, ufsrestore, and others, on volumes. You cannot use the format command, however. You can read, write, and copy files to and from a volume, as long as the volume contains a mounted file system.

Example—Volume That Consists of Two Slices

Figure 3–2 shows a volume that contains two slices, one slice from Disk A and one slice from Disk B. An application or UFS treats the volume as if it were one physical disk. Adding more slices to the volume increases its storage capacity.

Figure 3–2 Relationship Among a Volume, Physical Disks, and Slices

Diagram shows two disks, and how slices on those disks are presented by Solaris Volume Manager as a single logical volume.

Volume and Disk Space Expansion Using the growfs Command

Solaris Volume Manager enables you to expand a volume by adding additional slices. You can use either the Enhanced Storage tool within the Solaris Management Console or the command-line interface to add a slice to an existing volume.

You can expand a mounted or unmounted UFS file system that is contained within a volume without having to halt or back up your system. Nevertheless, backing up your data is always a good idea. After you expand the volume, use the growfs command to grow the file system.

Note –

After a file system has been expanded, the file system cannot be reduced in size. The inability to reduce the size of a file system is a UFS limitation. Similarly, after a Solaris Volume Manager partition has been increased in size, it cannot be reduced.

Applications and databases that use the raw volume must have their own method to “grow” the added space so that applications can recognize it. Solaris Volume Manager does not provide this capability.

You can expand the disk space in volumes in the following ways:

The growfs command expands a UFS file system without loss of service or data. However, write access to the volume is suspended while the growfs command is running. You can expand the file system to the size of the slice or the volume that contains the file system.

The file system can be expanded to use only part of the additional disk space by using the -s size option to the growfs command.

Note –

When you expand a mirror, space is added to the mirror's underlying submirrors. The growfs command is then run on the RAID-1 volume. The general rule is that space is added to the underlying devices, and the growfs command is run on the top-level device.

Volume Names

As with physical slices, volumes have logical names that appear in the file system. Logical volume names have entries in the /dev/md/dsk directory for block devices and the /dev/md/rdsk directory for raw devices. Instead of specifying the full volume name, such as /dev/md/dsk/volume-name, you can often use an abbreviated volume name, such as d1, with any meta* command. You can generally rename a volume, as long as the volume is not currently being used and the new name is not being used by another volume. For more information, see Exchanging Volume Names.

Originally, volume names had to begin with the letter “d” followed by a number (for example, d0). This format is still acceptable. The following are examples of volume names that use the “d*” naming construct:

/dev/md/dsk/d0

Block volume d0

/dev/md/dsk/d1

Block volume d1

/dev/md/rdsk/d126

Raw volume d126

/dev/md/rdsk/d127

Raw volume d127

Volume Name Guidelines

The use of a standard for your volume names can simplify administration and enable you at a glance to identify the volume type. Here are a few suggestions: