|Allocation Policy|| |
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.
|Disk Spanning|| |
The allocation of a logical volume across multiple
disks, allowing the volume size to exceed the size of a single disk.
|I/O Channel Separation|| |
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.
|Logical Extents|| |
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
|Logical Volume || |
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
|Logical Volume Manager || |
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.
|Physical Extents|| |
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.
|Physical Volume || |
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
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.
|Volume Group || |
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.