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Linux 2000 UK Linux Developers' Conference
Linux for the Enterprise
7 - 9 July 2000, Hammersmith (West London)

Heinz Mauelshagen

LVM - A Logical Volume Manager for Linux

Past, Present and Future

A Logical Volume Manager (LVM) is a subsystem for on-line disk storage management which has become a de-facto standard accross UNIX implementations and is a serious enabler for Linux in the Enterprise Computing area.

It adds an additional layer between the physical peripherals and the I/O interface in the kernel to get a logical view of disks. Unlike current partition schemes where disks are divided into fixed-sized sections, LVM allows the user to consider disks, also known as physical volumes (PV), as a pool (or volume) of data storage, consisting of equally-sized extents.

A LVM system consists of arbitrary groups of physical volumes, organized into volume groups (VG). A volume group can consist of one or more physical volumes. There can be more than one volume group in the system. Once created, the volume group, and not the disk, is the basic unit of data storage (think of it as a virtual disk consisting of one or more physical disks).

The pool of disk space that is represented by a volume group can be apportioned into virtual partitions, called logical volumes (LV) of various sizes. A logical volume can span a number of physical volumes or represent only a portion of one physical volume. The size of a logical volume is determined by its number of extents. Once created, logical volumes can be used like regular disk partitions - for eg. to create a file system or as a swap device.

Volume groups as well as logical volumes can be resized at runtime without system interruption (in the case of hot plugable disk subsystems) to enable continuous system uptime. The combination of the LVM with reiserfs for example enables the administrator to extend the logical volume and the filesytem stored on it while files are being accessed in it. In combination with database systems, newly created logical volumes can be used to address extended storage capacity needs.

Contents of logical volumes can be moved between physical volumes while the system is up without data loss. This lets you empty small or slow disks by moving their allocated extents to bigger and/or faster disk devices and then you can deconfigure them from the volume group they belong to. Another option is to move i/o intensive extents to less-used disk devices in the same volume group.

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