Glossary

The LVM allocation policy governing how disk space is distributed to logical volumes and how extents are laid out on an LVM disk. LVM allocates disk space in terms of strict vs. non-strict and contiguous vs. noncontiguous. Strict allocation requires that mirror copies reside on different LVM disks. Contiguous allocation requires that no gaps exist between physical extents on a single disk.

The allocation of a logical volume across multiple disks, allowing the volume size to exceed the size of a single disk.

A configuration of disks useful for segregating highly I/O-intensive areas. For example, you might have a database on one channel and file systems on another. When mirroring logical volumes using HP MirrorDisk/UX, you can spread the mirrored copies over different I/O channels to increase system and data availability.

Fixed-size addressable areas of space on a logical volume. The basic allocation unit for a logical volume, a logical extent is mapped to a physical extent; thus, if the physical extent size is 4 MB, the logical extent size will also be 4 MB. The size of a logical volume is determined by the number of logical extents configured.

A virtual storage device of flexible size that can hold a file system, raw data, dump area, or swap. Because its data are distributed logically (rather than physically), a single logical volume can be mapped to one LVM disk or span multiple disks. A logical volume appears to the administrator as though it was a single disk.

An operating system software module that implements virtual (logical) disks to extend, mirror, and improve the performance of physical disk access.

Simultaneous replication of data, ensuring a greater degree of data availability. LVM can map identical logical volumes to multiple LVM disks, thus providing the means to recover easily from the loss of one copy (or multiple copies in the case of multi-way mirroring) of data. Mirroring can provide faster access to data for applications using more data reads than writes. Mirroring requires the MirrorDisk/UX product.

Fixed-size addressable areas of space on an LVM disk. They are the basic allocation units for a physical volume. Physical extents map to areas on logical volumes called logical extents.

A disk that has been initialized by LVM for inclusion in a volume group; also called an LVM disk. As with standard disks, an LVM disk (physical volume) is accessed via a raw device file (for example, /dev/rdisk/disk3). Use the HP SMH or the pvcreate command to initialize a disk as a physical volume.

A subset of physical volumes within a volume group, each with a separate I/O channel or interface adapter to achieve higher availability of mirrored data.

The requirement that a certain number of LVM disks be present in order to change or activate a volume group. To activate a volume group, quorum requires the number of available LVM disks to be more than half the number of configured LVM disks that were present when the volume group was last active. To make a configuration change, the quorum requirement is at least half. If there is no quorum, LVM prevents the operation. Quorum is checked both during configuration changes (for example, when creating a logical volume) and at state changes (for example, if a disk fails). Quorum ensures the consistency and integrity of the volume groups. The vgchange command with the -q n option can be used to override quorum check, but this should be used with caution.

The process of updating stale (non-current) copies of mirrored logical extents by copying data from a fresh (current) copy of the logical volume. Synchronization keeps mirrored logical volumes consistent by ensuring that all copies contain the same data.

A collection of one or more LVM disks from which disk space may be allocated to individual logical volumes. A disk can belong to only one volume group. A volume group is accessed through the group file (for example, /dev/vg01/group) in that volume group's directory. Use HP SMH or the vgcreate command to create a volume group.