Permanent data storage normally uses hard disk drive (HDD) technology originally developed by IBM in the mid 1950s. Today, HDDs are typically aggregated into arrays in storage appliances using various methods to provide redundancy in case of the failure of any single HDD. In this case, the array, or a portion of the storage space in the array, will be presented to a client operating system (OS) as a logical unit (LUN). This is not yet logical volume management (LVM), however, the LUN still appears to the OS as a single, large-capacity storage device. For LVM purposes, there is no functional difference between a local HDD and a LUN, so we will simply call either type of storage a disk.
LVM use arises when we consider how the client OS is going to subdivide the space in a disk for its use. This subdivision is necessary as disk space may be needed for different things such as page space, file systems, a kernel dump device or possibly raw data space for an application that does not want to use files to store its data.
The majority of disk space is normally used for file storage, and there are many reasons to implement multiple file systems to store data in categories that are best handled differently. Each file system will then need to have space allotted from the disks available.
Early operating systems solved this problem by disk partitioning. This involved dividing each disk into a few fixed-size partitions, whereby beginning and end points were defined by a partition table written to a reserved portion of each disk. This worked but it had limitations. The size of the partition table typically limited the number of partitions to eight or less. Also, as the partition table was read and stored in memory at boot, changes to it typically required either a reboot or at least the removal of the disk from production to reconfigure it. Either case required data to go out of production.
Finally, partitions could not be dynamically resized, making it necessary to try to predict how much space would be needed in a partition as time passed. This was not always easy and resulted in partitions that were either too large or too small. In this case, restructuring of disk partition tables was necessary, disrupting production.
The 1989 release of AIXv3 introduced the mandatory use of LVM to address these limitations.
The Concept of Logical Volume Management
If partition tables divide disks into a small number of large partitions, the LVM reverses this idea, dividing disks into a large number of small partitions. To distinguish the difference, LVM schemes use the term physical extent, or in the case of AIX, physical partition (PP) to describe these small units of disk space. The disk itself is called a physical volume (PV). When multiple PVs are available they can be collected into volume groups (VGs).
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