Introduction; Lvm Features - HP -UX 11i Administrator's Manual

1 Introduction

This chapter addresses the following topics:
"LVM Features" (page 8)
"LVM Architecture" (page 9)
"Physical versus Logical Extents" (page 10)
"LVM Volume Group Versions" (page 11)
"LVM Device File Usage" (page 12)
"LVM Disk Layout" (page 16)
"LVM Limitations" (page 18)
"Shared LVM" (page 18)

LVM Features

Logical Volume Manager (LVM) is a storage management system that lets you allocate and manage
disk space for file systems or raw data. Historically, you treated your disks individually and in
terms of fixed-sized partitions; each disk or partition held a file system, swap space, boot area,
or raw data. With LVM, you do not need to assign a disk or fixed-sized partition to a single
purpose. Instead, consider the disks as a pool (or volume) of data storage, consisting of equal-sized
extents. Extents are allocated into virtual storage devices known as
treated as disks.
LVM provides the following capabilities:
A logical volume size can be dynamically reduced or expanded to meet changing data needs.
For example, a logical volume can be as small or large as the file system mounted to it requires.
The file system can be extended without rebuilding it or the logical volume; reducing a file
system is more complex, and may require recreating the file system.
Small chunks of unused space from several disks can be combined to create a usable volume.
A logical volume can exceed the size of a single physical disk. This feature is called
spanning, because a single file system (and individual files) can span disks.
Up to six copies of identical data can be stored and updated simultaneously using LVM. This
feature is called
MirrorDisk/UX. See
Mirrored data can be configured to automatically create a new mirror to a separate disk
when one of the mirror copies fails. This feature is called sparing, and requires an optional
product, HP MirrorDisk/UX. See
(page 27).
A logical volume can be created so that logically contiguous data blocks (for example, chunks
of the same file) are distributed across multiple disks, which speeds I/O throughput for large
files when they are read and written sequentially. This feature is called striping. Striping can
be used in conjunction with mirroring. See
(page 31).
A point-in-time image of a logical volume can be created, called a snapshot logical volume.
See "Creating and Administering Snapshot Logical Volumes" (page
Devices accessed through multiple links can be configured to improve availability. If the primary
link to a device fails, LVM can switch automatically to an alternate link. This feature is called
multipathing. See
8 Introduction
mirroring a logical volume, and requires an optional product, HP
"Increasing Data Availability Through Mirroring" (page
"Increasing Disk Redundancy Through Disk Sparing"
"Increasing Hardware Path Redundancy Through Multipathing" (page
logical
"Increasing Performance Through Disk Striping"
volumes, which can be
disk
24).
103).
28).