A logical volume is a name for a group of physical slices that appear to the system as a single device. These may be called pseudo or virtual devices in standard UNIX terms.
You can create a logical volume to expand storage space (using RAID 0 volumes, which concatenate or stripe slices together) or to provide redundancy (using RAID 1 volumes, which mirror data onto two or more slices), or to do both (using RAID 5 volumes, which use one disk of a set to store parity information). You can also combine different types of volumes to achieve specific goals.
Note: Additionally, transactional volumes provide improved data availability by eliminating the need to fsck disks after a crash--this is the same function that UFS logging provides. You can use either Enhanced Storage or UFS for this purpose, but UFS logging generally offers superior performance.
You use volumes to increase storage capacity 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, 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 Conventions for details about volume names.
You can use most file systems commands (mount(1M), umount(1M), ufsdump(1M), ufsrestore(1M),and so forth) on volumes. You cannot use the format(1M) command, however. You can read, write, and copy files to and from a volume, as long as you have a file system mounted on the volume.
The following table summarizes the types of logical volumes:
Types of Volumes
Volume |
Description |
---|---|
RAID 0 (Concatenations and stripes) |
Can be used directly, or as the basic building blocks for mirrors and transactional devices. There are three types of RAID 0 volumes: stripes, concatenations, and concatenated stripes. RAID 0 volumes consist only of physical slices. By themselves, simple volumes do not provide data redundancy. |
RAID 1 (Mirrors) |
Replicates data by maintaining multiple copies. A mirror is composed of one or more RAID 0 volumes called submirrors. |
RAID 5 |
Replicates data by using parity information. In the case of missing data, the missing data can be regenerated using available data and the parity information. A RAID 5 volume is composed of slices. One slice's worth of space is allocated to parity information, but it is distributed across all slices in the RAID 5 volume. |
Transactional volumes |
Used to log a file system. A transactional volume is composed of a master device and a logging device. Both of these devices can be a slice, simple volume, mirror, or RAID 5 volume. The master device contains the logged file system. |