Lvm Architecture - HP -UX 11i Administrator's Manual

LVM Architecture

An LVM system starts by initializing disks for LVM usage. An LVM disk is known as a physical
volume (PV). A disk is marked as an LVM physical volume using either the HP System Management
Homepage (HP SMH) or the pvcreate command. Physical volumes use the same device special
files as traditional HP-UX disk devices.
LVM divides each physical volume into addressable units called physical extents (PEs). Starting
after the LVM metadata at the beginning of the disk, extents are allocated sequentially, with an
index starting at zero and incrementing by one for each unit. The physical extent size is configurable
at the time you form a volume group and applies to all disks in the volume group. You can select
a size from 1 MB to 256 MB.
Physical volumes are organized into volume groups (VGs). A volume group can consist of one or
more physical volumes, and there can be more than one volume group in the system. Once created,
the volume group, not the disk, is the entity that represents data storage. Thus, whereas earlier you
moved disks from one system to another, with LVM, you move a volume group from one system to
another. Therefore, it is often convenient to have multiple volume groups on a system.
The pool of disk space that is represented by a volume group can be divided into logical volumes
(LVs) of various sizes. Once created, logical volumes can be treated just like disk partitions. They
are accessible through device special files. A logical volume can span a number of physical volumes
in a volume group or represent only part of one physical volume.
The basic allocation units for a logical volume are called logical extents (LEs). A logical extent is
mapped to a physical extent. Thus, if the physical extent size is 4 MB, the logical extent size is
also 4 MB. The size of a logical volume is determined by the number of logical extents configured.
Starting with volume group Version 2.2, LVM introduced a new type of logical volume: snapshot
logical volume. Snapshots are point-in-time image of a logical volume. Snapshots allow you to
create another copy of the logical volume (which can be used for a backup) without taking up as
much of the physical space as the size of the logical volume. See "Creating and Administering
Snapshot Logical Volumes" (page 103) for more information about snapshots.
You assign file systems, swap, dump, or raw data to logical volumes. For example, in Figure 1,
logical volume /dev/vg01/lvol1 might contain a file system, logical volume /dev/vg01/
lvol2 might contain swap space, and logical volume /dev/vg01/lvol3 might contain raw
data. You can use HP SMH to create a file system in a logical volume of a specified size, then
mount the file system. Alternately, you can use LVM commands to create, then extend a logical
volume to allocate sufficient space for file systems or raw data. You then create and mount new
file systems or install your application in the logical volume.
LVM Architecture 9