Storage management concepts

The fundamental concepts used by LVM are physical volumes, volume groups,physical partitions, logical volumes, logical partitions, file systems, and rawdevices. Some of their characteristics are presented as follows: Each individual disk drive is a named physical volume (PV) and has a namesuch as hdisk0 or hdisk1. One or more PVs can make up a volume group (VG). A physical volume canbelong to a maximum of one VG. You cannot assign a fraction of a PV to one VG. A physical volume isassigned entirely to a volume group. Physical volumes can be assigned to the same volume group even thoughthey 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 thesame 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 pointof view, but can be spread on different physical volumes from the samevolume group. Logical volumes consist of one or more logical partitions (LPs). Each logicalpartition has at least one corresponding physical partition. A logical partitionand a physical partition always have the same size. You can have up to threecopies of the data located on different physical partitions. Usually, physicalpartitions storing identical data are located on different physical disks forredundancy purposes. Data from a logical volume can be stored in an organized manner, having theform of files located in directories. This structured and hierarchical form oforganization 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 theresponsibility of the application that uses this data to access and interpret itcorrectly. The volume group descriptor area (VGDA) is an area on the disk that containsinformation pertinent to the volume group that the physical volume belongs to.It also includes information about the properties and status of all physical andlogical volumes that are part of the volume group. The information from VGDAis used and updated by LVM commands. There is at least one VGDA perphysical volume. Information from VGDAs of all disks that are part of thesame volume group must be identical. The VGDA internal architecture andChapter 6. Disk storage management 213location on the disk depends on the type of the volume group (original, big, orscalable). The volume group status area (VGSA) is used to describe the state of allphysical partitions from all physical volumes within a volume group. TheVGSA 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 diskdepends on the type of the volume group. A logical volume control block (LVCB) contains important information aboutthe logical volume, such as the number of the logical partitions or diskallocation policy. Its architecture and location on the disk depends on the typeof the volume group it belongs to. For standard volume groups, the LVCBresides 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 volumegroups, all relevant logical volume control information is kept in the VGDA aspart of the LVCB information area and the LV entry area.