A volume group (VG) is a logical collection, or purposeful grouping, of computer storage space that may span multiple physical devices. It combines both physical and logical volumes into a single administrative element within UNIX® and LINUX® logical volume management (LVM). A physical volume may be an internal or external storage device. Logical volumes are concatenations, or chained lists, of storage space that might join portions of several physical volumes. Under LINUX®, the volume group is the highest level of abstraction, or visualization, in its logical volume manager utility.
In early implementations, physical disks were made available to operating systems by means of static partitions. These partitions may be a sequential portion or the entirety of a single physical device. Each partition would appear to be a separate drive to the operating system. If a partition needed more space, the information held there would have to be transferred to a larger partition on the same or another physical drive.
Many systems, particularly those based on UNIX®, now break up space on physical disks into standard sized storage units. Logical volumes made up of units from multiple drives can be allocated to partitions, and storage units can be added or removed as requirements change. This is the foundation of logical volume management.
Physical volumes can be hard disks, partitions of a disk or an external storage device. They are considered a concatenated series of storage spaces called physical extents (PE). These are the basic units of storage allocation and, depending on the system, may be of uniform or variable size. Logical volumes are similarly made up of logical extents (LE), which are same size as the physical extents. The role of the volume group is to map the logical extents to physical extents, creating a pool of LEs that can be joined into logical volumes, or virtual partitions.
Logical volumes can be enlarged or reduced in size as needed simply by adding LEs or returning them to them pool. The LEs do not have to be sequential or even share the same device. A volume group can change size with the addition or removal of a storage drive, though this may necessitate the movement or reassignment of LEs. Physical and logical volumes are unique to their volume group, however, and cannot span or be shared by groups. As an administrative unit, a volume group itself can be moved to another host machine, made subject to access permissions or hidden entirely.
