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Volume Groups
may be necessary to move data around partitions when you are migrating an existing system to
LVM volumes. Additionally, if you have a very large disk and want to have more than one
volume group for administrative purposes then it is necessary to partition the disk. If you do
have a disk with more than one partition and both of those partitions are in the same volume
group, take care to specify which partitions are to be included in a logical volume when creating
striped volumes.
2. Volume Groups
Physical volumes are combined into volume groups (VGs). This creates a pool of disk space out
of which logical volumes can be allocated.
Within a volume group, the disk space available for allocation is divided into units of a fixed-size
called extents. An extent is the smallest unit of space that can be allocated, Within a physical
volume, extents are referred to as physical extents.
A logical volume is allocated into logical extents of the same size as the physical extents. The
extent size is thus the same for all logical volumes in the volume group. The volume group
maps the logical extents to physical extents.
3. LVM Logical Volumes
In LVM, a volume group is divided up into logical volumes. There are three types of LVM logical
volumes: linear volumes, striped volumes, and mirrored volumes. These are described in the
following sections.
3.1. Linear Volumes
A linear volume aggregates multiple physical volumes into one logical volume. For example, if
you have two 60GB disks, you can create a 120GB logical volume. The physical storage is
concatenated.
Creating a linear volume assigns a range of physical extents to an area of a logical volume in
order. For example, as shown in
Figure 2.2, "Extent Mapping"
logical extents 1 to 99 could map
to one physical volume and logical extents 100 to 198 could map to a second physical volume.
From the point of view of the application, there is one device that is 198 extents in size.
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