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Logical Volume Manager

Introduction

Logical Volume Manager (LVM) is a storage management system that is part of the Linux kernel. It can be thought of as a layer of software that exists between the physical storage disk/partitions and the operating system.

Overview

For any explanation of LVM to make sense it is first essential to understand how Linux handles storage devices. As part of the “Everything is a File” philosophy the hard disks can be accessed through files. For example, /dev/sda is the file reference for Scsi Disk 1. Linux distinguishes between disks using letters and differentiates partitions using numbers. With that in mind /dev/sda1 is the first partition of the first scsi disk.

If my system contains three 1TB hard drives with 2 500GB partitions each I will have:

 /dev/sda1
 /dev/sda2
 /dev/sdb1
 /dev/sdb2

I can create file systems on each of them, mount them in different locations. If those needs/requirements regarding sizing were to change I would have a problem. LVM addresses many of the common needs that non-LVM systems lack or poorly support.

LVM is made up of three main components.

 Physical Volumes
 Volume Groups
 Logical Volumes

A Physical Volume (PV) can be a hard disk, or a hard disk partition. Either /dev/sda or /dev/sda1 will work. Once a disk/partition has been defined as a PV it can be used by LVM and added to a Volume Group (VG).

A VG is a collection of PVs that are presented to the operating system as one continuous disk. Now, LVM can work with the VG and cut it up in to smaller units called Logical Volumes (LV). From the operating systems point of view an LV can be thought of as a disk partition. It can have a file system, it can be mounted.

Features

Why go through all that trouble if you're just going to end up with LVs that are similar to partitions anyway? I said earlier that LVM lets us do things disk partitions cannot. LVM can:

 Resize Logical Volumes 
 Resize Volume Groups
 Create snapshots of Logical Volumes
 Mirror Logical Volumes
 Stripe Logical Volumes
 Move Logical Volumes across Physical Volumes
 
All without bringing the system offline or requiring a reboot.


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