Logical volume storage

Logical volumes are groups of information located on physical volumes.

A hierarchy of structures is used to manage disk storage. Each individual disk drive, called a physical volume (PV) has a name, such as /dev/hdisk0. Every physical volume in use belongs to a volume group (VG). All of the physical volumes in a volume group are divided into physical partitions (PPs) of the same size. For space-allocation purposes, each physical volume is divided into five regions (outer_edge, inner_edge, outer_middle, inner_middle and center). The number of physical partitions in each region varies, depending on the total capacity of the disk drive.

Within each volume group, one or more logical volumes (LVs) are defined. Data on logical volumes appears to be contiguous to the user but can be discontiguous on the physical volume. This allows file systems, paging space, and other logical volumes to be re-sized or relocated, to span multiple physical volumes, and to have their contents replicated for greater flexibility and availability in the storage of data.

Each logical volume consists of one or more logical partitions (LPs). Each logical partition corresponds to at least one physical partition. If mirroring is specified for the logical volume, additional physical partitions are allocated to store the additional copies of each logical partition. Although the logical partitions are numbered consecutively, the underlying physical partitions are not necessarily consecutive or contiguous.

Logical volumes can serve a number of system purposes, such as paging, but each logical volume serves a single purpose only. Many logical volumes contain a single journaled file system (JFS or JFS2). Each JFS consists of a pool of pagesize (4 KB) blocks. When data is to be written to a file, one or more additional blocks are allocated to that file. These blocks might not be contiguous with one another or with other blocks previously allocated to the file. A given file system can be defined as having a fragment size of less than 4 KB (512 bytes, 1 KB, 2 KB).

After installation, the system has one volume group (the rootvg volume group) consisting of a base set of logical volumes required to start the system and any other logical volumes you specify to the installation script. Any other physical volumes you have connected to the system can be added to a volume group (using the extendvg command). You can add the physical volume either to the rootvg volume group or to another volume group (defined by using the mkvg command). Logical volumes can be tailored using the commands, the menu-driven System Management Interface Tool (SMIT) interface.