Volume groups

A volume group is a collection of 1 to 32 physical volumes of varying sizes and types.
A big volume group can have from 1 to 128 physical volumes. A scalable volume group can have up to 1024 physical volumes. A physical volume can belong to only one volume group per system; there can be up to 255 active volume groups.
When a physical volume is assigned to a volume group, the physical blocks of storage media on it are organized into physical partitions of a size you specify when you create the volume group. For more information.
When you install the system, one volume group (the root volume group, called rootvg) is automatically created that contains the base set of logical volumes required to start the system, as well as any other logical volumes you specify to the installation script. The rootvg includes paging space, the journal log, boot data, and dump storage, each in its own separate logical volume. The rootvg has attributes that differ from user-defined volume groups. For example, the rootvg cannot be imported or exported. When performing a command or procedure on the rootvg, you must be familiar with its unique characteristics.
You create a volume group with the mkvg command. You add a physical volume to a volume group with the extendvg command, make use of the changed size of a physical volume with the chvg command, and remove a physical volume from a volume group with the reducevg command. Some of the other commands that you use on volume groups include: list (lsvg), remove (exportvg), install (importvg), reorganize (reorgvg), synchronize (syncvg), make available for use (varyonvg), and make unavailable for use (varyoffvg).
Small systems might require only one volume group to contain all the physical volumes attached to the system. You might want to create separate volume groups, however, for security reasons, because each volume group can have its own security permissions. Separate volume groups also make maintenance easier because groups other than the one being serviced can remain active. Because the rootvg must always be online, it contains only the minimum number of physical volumes necessary for system operation.
You can move data from one physical volume to other physical volumes in the same volume group with the migratepv command. This command allows you to free a physical volume so it can be removed from the volume group. For example, you could move data from a physical volume that is to be replaced.
A volume group that is created with smaller physical and logical volume limits can be converted to a format which can hold more physical volumes and more logical volumes. This operation requires that there be enough free partitions on every physical volume in the volume group for the volume group descriptor area (VGDA) expansion. The number of free partitions required depends on the size of the current VGDA and the physical partition size. Because the VGDA resides on the edge of the disk and it requires contiguous space, the free partitions are required on the edge of the disk. If those partitions are allocated for a user's use, they are migrated to other free partitions on the same disk. The rest of the physical partitions are renumbered to reflect the loss of the partitions for VGDA usage. This renumbering changes the mappings of the logical to physical partitions in all the physical volumes of this volume group. If you have saved the mappings of the logical volumes for a potential recovery operation, generate the maps again after the completion of the conversion operation. Also, if the backup of the volume group is taken with map option and you plan to restore using those maps, the restore operation might fail because the partition number might no longer exist (due to reduction). It is recommended that backup is taken before the conversion and right after the conversion if the map option is used. Because the VGDA space has been increased substantially, every VGDA update operation (creating a logical volume, changing a logical volume, adding a physical volume, and so on) might take considerably longer to run.