Storage management concepts

The fundamental concepts used by LVM are physical volumes, volume groups,
physical partitions, logical volumes, logical partitions, file systems, and raw
devices. Some of their characteristics are presented as follows:
 Each individual disk drive is a named physical volume (PV) and has a name
such as hdisk0 or hdisk1.
 One or more PVs can make up a volume group (VG). A physical volume can
belong to a maximum of one VG.
 You cannot assign a fraction of a PV to one VG. A physical volume is
assigned entirely to a volume group.
 Physical volumes can be assigned to the same volume group even though
they are of different types, such as SCSI or SSA.
 Storage space from physical volumes is divided into physical partitions (PPs).
The size of the physical partitions is identical on all disks belonging to the
same VG.
 Within each volume group, one or more logical volumes (LVs) can be defined.
Data stored on logical volumes appears to be contiguous from the user point
of view, but can be spread on different physical volumes from the same
volume group.
 Logical volumes consist of one or more logical partitions (LPs). Each logical
partition has at least one corresponding physical partition. A logical partition
and a physical partition always have the same size. You can have up to three
copies of the data located on different physical partitions. Usually, physical
partitions storing identical data are located on different physical disks for
redundancy purposes.
 Data from a logical volume can be stored in an organized manner, having the
form of files located in directories. This structured and hierarchical form of
organization is named a file system.
 Data from a logical volume can also be seen as a sequential string of bytes.
This type of logical volumes are named raw logical volumes. It is the
responsibility of the application that uses this data to access and interpret it
correctly.
 The volume group descriptor area (VGDA) is an area on the disk that contains
information pertinent to the volume group that the physical volume belongs to.
It also includes information about the properties and status of all physical and
logical volumes that are part of the volume group. The information from VGDA
is used and updated by LVM commands. There is at least one VGDA per
physical volume. Information from VGDAs of all disks that are part of the
same volume group must be identical. The VGDA internal architecture and
Chapter 6. Disk storage management 213
location on the disk depends on the type of the volume group (original, big, or
scalable).
 The volume group status area (VGSA) is used to describe the state of all
physical partitions from all physical volumes within a volume group. The
VGSA indicates if a physical partition contains accurate or stale information.
VGSA is used for monitoring and maintained data copies synchronization.
The VGSA is essentially a bitmap and its architecture and location on the disk
depends on the type of the volume group.
 A logical volume control block (LVCB) contains important information about
the logical volume, such as the number of the logical partitions or disk
allocation policy. Its architecture and location on the disk depends on the type
of the volume group it belongs to. For standard volume groups, the LVCB
resides on the first block of user data within the LV. For big volume groups,
there is additional LVCB information in VGDA on the disk. For scalable volume
groups, all relevant logical volume control information is kept in the VGDA as
part of the LVCB information area and the LV entry area.