The Logical Volume Manager controls disk resources by
mapping data between a simple and flexible logical view of storage space and
the actual physical disks. The Logical Volume Manager does this by using a
layer of device driver code that runs above the traditional physical device
drivers. This logical view of the disk storage is provided to applications and is
independent of the underlying physical disk structure.

Each individual disk drive is called a physical volume (PV) and has a name,
usually /dev/hdiskx (where x is a unique integer on the system). Every
physical volume in use belongs to a volume group (VG) unless it is being
used as a raw storage device or a readily available spare (often called a ’Hot
Spare’). Each physical volume consists of a number of disks (or platters)
stacked one above the other. Each is divided into physical partitions (PPs) of
a fixed size for that physical volume. For space allocation purposes, each
physical volume is divided into five regions (outer_edge, outer_middle,
center, inner_middle, and inner_edge), and these can be viewed as
cylindrical segments cut perpendicularly through the disk platters.

The set of operating system commands, library subroutines, and other tools
that allow the user to establish and control logical volume storage is called
the Logical Volume Manager. The Logical Volume Manager controls disk resources by mapping data between a simple and flexible logical view of storage space and the actual physical disks.

Within each volume group, one or more logical volumes (LVs) are defined.
Logical volumes are the way to group information located on one or more
physical volumes. Logical volumes are an area of disk used to store data,
which appears to be contiguous to the application, but can be non-contiguous
on the actual physical volume. It is this definition of a logical volume that
allows them to be extended, relocated, span multiple physical volumes, and
have their contents replicated for greater flexibility and availability.

Physical volumes
A storage device that are divided into physical
partitions.

Volume groups
A collection of one or more physical volumes,
independent of type.

Logical volumes
A collection of logical partitions, each of which can
be mapped to any physical partition in the volume
group. If mirroring is used, the logical partition is
mapped to two or three physical partitions.

Logical volume manager
Controls all this through the logical volume device
driver. It takes a complex structure of physical
partitions, including mirroring, and presents a
simple logical partitions structure to the
user/application.

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