Red Hat ENTERPRISE LINUX 4 - LVM ADMINISTRATOR, Enterprise Linux 4 User Manual

Red Hat Enterprise Linux 4

Cluster Logical

Volume Manager

LVM Administrator's Guide

Cluster Logical Volume Manager

Red Hat Enterprise Linux 4 Cluster Logical Volume Manager
LVM Administrator's Guide
Edition 1.0

Copyright © 2009 Red Hat Inc.. This material may only be distributed subject to the terms and
conditions set forth in the Open Publication License, V1.0 or later (the latest version of the OPL is
presently available at http://www.opencontent.org/openpub/).

Red Hat and the Red Hat "Shadow Man" logo are registered trademarks of Red Hat, Inc. in the United
States and other countries.

All other trademarks referenced herein are the property of their respective owners.

1801 Varsity Drive
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Phone: [phone]
Fax: [phone]
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This book describes the LVM logical volume manager, including information on running LVM in a
clustered environment. The content of this document is specific to the LVM2 release.

iii

Introduction vii

1. Audience ....................................................................................................................... vii

2. Software Versions .......................................................................................................... vii

3. Related Documentation .................................................................................................. vii

4. Document Conventions ................................................................................................. viii

4.1. Typographic Conventions ................................................................................... viii

4.2. Pull-quote Conventions ......................................................................................... x

4.3. Notes and Warnings ............................................................................................. x

5. Feedback ....................................................................................................................... xi

1. The LVM Logical Volume Manager 1

1.1. Logical Volumes ........................................................................................................... 1

1.2. LVM Architecture Overview ........................................................................................... 2

1.3. Running LVM in a Cluster ............................................................................................ 3

1.4. Document Overview ..................................................................................................... 4

2. LVM Components 5

2.1. Physical Volumes ......................................................................................................... 5

2.1.1. LVM Physical Volume Layout ............................................................................. 5

2.1.2. Multiple Partitions on a Disk ............................................................................... 6

2.2. Volume Groups ............................................................................................................ 6

2.3. LVM Logical Volumes ................................................................................................... 7

2.3.1. Linear Volumes ................................................................................................. 7

2.3.2. Striped Logical Volumes .................................................................................... 8

2.3.3. Mirrored Logical Volumes .................................................................................. 9

2.3.4. Snapshot Volumes ........................................................................................... 10

3. LVM Administration Overview 13

3.1. Creating LVM Volumes in a Cluster ............................................................................. 13

3.2. Logical Volume Creation Overview .............................................................................. 13

3.3. Growing a File System on a Logical Volume ................................................................ 14

3.4. Logical Volume Backup .............................................................................................. 14

3.5. Logging ...................................................................................................................... 14

4. LVM Administration with CLI Commands 17

4.1. Using CLI Commands ................................................................................................ 17

4.2. Physical Volume Administration ................................................................................... 18

4.2.1. Creating Physical Volumes ............................................................................... 19

4.2.2. Displaying Physical Volumes ............................................................................ 20

4.2.3. Preventing Allocation on a Physical Volume ...................................................... 21

4.2.4. Resizing a Physical Volume ............................................................................. 21

4.2.5. Removing Physical Volumes ............................................................................ 21

4.3. Volume Group Administration ...................................................................................... 21

4.3.1. Creating Volume Groups .................................................................................. 22

4.3.2. Adding Physical Volumes to a Volume Group .................................................... 23

4.3.3. Displaying Volume Groups ............................................................................... 23

4.3.4. Scanning Disks for Volume Groups to Build the Cache File ................................ 24

4.3.5. Removing Physical Volumes from a Volume Group ............................................ 24

4.3.6. Changing the Parameters of a Volume Group ................................................... 25

4.3.7. Activating and Deactivating Volume Groups ...................................................... 25

4.3.8. Removing Volume Groups ............................................................................... 26

4.3.9. Splitting a Volume Group ................................................................................. 26

4.3.10. Combining Volume Groups ............................................................................. 26

Cluster Logical Volume Manager

iv

4.3.11. Backing Up Volume Group Metadata .............................................................. 26

4.3.12. Renaming a Volume Group ............................................................................ 27

4.3.13. Moving a Volume Group to Another System .................................................... 27

4.3.14. Recreating a Volume Group Directory ............................................................. 28

4.4. Logical Volume Administration .................................................................................... 28

4.4.1. Creating Logical Volumes ................................................................................ 28

4.4.2. Persistent Device Numbers .............................................................................. 32

4.4.3. Resizing Logical Volumes ................................................................................ 33

4.4.4. Changing the Parameters of a Logical Volume Group ........................................ 33

4.4.5. Renaming Logical Volumes .............................................................................. 33

4.4.6. Removing Logical Volumes .............................................................................. 33

4.4.7. Displaying Logical Volumes .............................................................................. 34

4.4.8. Growing Logical Volumes ................................................................................. 34

4.4.9. Extending a Striped Volume ............................................................................. 35

4.4.10. Shrinking Logical Volumes ............................................................................. 37

4.5. Creating Snapshot Volumes ........................................................................................ 37

4.6. Controlling LVM Device Scans with Filters ................................................................... 38

4.7. Online Data Relocation ............................................................................................... 39

4.8. Activating Logical Volumes on Individual Nodes in a Cluster ......................................... 40

4.9. Customized Reporting for LVM ................................................................................... 40

4.9.1. Format Control ................................................................................................ 41

4.9.2. Object Selection .............................................................................................. 43

4.9.3. Sorting LVM Reports ....................................................................................... 49

4.9.4. Specifying Units ............................................................................................... 50

5. LVM Configuration Examples 53

5.1. Creating an LVM Logical Volume on Three Disks ......................................................... 53

5.1.1. Creating the Physical Volumes ......................................................................... 53

5.1.2. Creating the Volume Group .............................................................................. 53

5.1.3. Creating the Logical Volume ............................................................................ 53

5.1.4. Creating the File System .................................................................................. 54

5.2. Creating a Striped Logical Volume .............................................................................. 54

5.2.1. Creating the Physical Volumes ......................................................................... 54

5.2.2. Creating the Volume Group .............................................................................. 55

5.2.3. Creating the Logical Volume ............................................................................ 55

5.2.4. Creating the File System .................................................................................. 55

5.3. Splitting a Volume Group ............................................................................................ 56

5.3.1. Determining Free Space .................................................................................. 56

5.3.2. Moving the Data .............................................................................................. 56

5.3.3. Splitting the Volume Group .............................................................................. 57

5.3.4. Creating the New Logical Volume ..................................................................... 57

5.3.5. Making a File System and Mounting the New Logical Volume ............................. 57

5.3.6. Activating and Mounting the Original Logical Volume ......................................... 58

5.4. Removing a Disk from a Logical Volume ..................................................................... 58

5.4.1. Moving Extents to Existing Physical Volumes .................................................... 58

5.4.2. Moving Extents to a New Disk .......................................................................... 59

5.5. Creating a Mirrored LVM Logical Volume in a Cluster ................................................... 61

6. LVM Troubleshooting 65

6.1. Troubleshooting Diagnostics ....................................................................................... 65

6.2. Displaying Information on Failed Devices ..................................................................... 65

6.3. Recovering from LVM Mirror Failure ............................................................................ 66

v

6.4. Recovering Physical Volume Metadata ........................................................................ 70

6.5. Replacing a Missing Physical Volume .......................................................................... 71

6.6. Removing Lost Physical Volumes from a Volume Group ............................................... 72

6.7. Insufficient Free Extents for a Logical Volume .............................................................. 72

7. LVM Administration with the LVM GUI 75

A. The Device Mapper 77

B. The LVM Configuration Files 79

B.1. The LVM Configuration Files ...................................................................................... 79

B.2. Sample lvm.conf File .................................................................................................. 79

C. LVM Object Tags 89

C.1. Adding and Removing Object Tags ............................................................................. 89

C.2. Host Tags ................................................................................................................. 89

C.3. Controlling Activation with Tags .................................................................................. 90

D. LVM Volume Group Metadata 91

D.1. The Physical Volume Label ........................................................................................ 91

D.2. Metadata Contents ..................................................................................................... 91

D.3. Sample Metadata ....................................................................................................... 92

E. Revision History 95

Index 97

vi

vii

Introduction

This book describes the Logical Volume Manager (LVM), including information on running LVM in a
clustered environment. The content of this document is specific to the LVM2 release.

1. Audience

This book is intended to be used by system administrators managing systems running the Linux
operating system. It requires familiarity with Red Hat Enterprise Linux and GFS file system
administration.

2. Software Versions

Software Description

RHEL4 refers to RHEL4 and higher

GFS refers to GFS 6.1 and higher

Table 1. Software Versions

3. Related Documentation

For more information about using Red Hat Enterprise Linux, refer to the following resources:

• Red Hat Enterprise Linux Installation Guide — Provides information regarding installation of Red
Hat Enterprise Linux.

• Red Hat Enterprise Linux Introduction to System Administration — Provides introductory information
for new Red Hat Enterprise Linux system administrators.

• Red Hat Enterprise Linux System Administration Guide — Provides more detailed information about
configuring Red Hat Enterprise Linux to suit your particular needs as a user.

• Red Hat Enterprise Linux Reference Guide — Provides detailed information suited for more
experienced users to reference when needed, as opposed to step-by-step instructions.

• Red Hat Enterprise Linux Security Guide — Details the planning and the tools involved in creating a
secured computing environment for the data center, workplace, and home.

For more information about Red Hat Cluster Suite for Red Hat Enterprise Linux, refer to the following
resources:

• Red Hat Cluster Suite Overview — Provides a high level overview of the Red Hat Cluster Suite.

• Configuring and Managing a Red Hat Cluster — Provides information about installing, configuring
and managing Red Hat Cluster components.

• Global File System: Configuration and Administration — Provides information about installing,
configuring, and maintaining Red Hat GFS (Red Hat Global File System).

• Using Device-Mapper Multipath — Provides information about using the Device-Mapper Multipath
feature of Red Hat Enterprise Linux.

Introduction

viii

• Using GNBD with Global File System — Provides an overview on using Global Network Block
Device (GNBD) with Red Hat GFS.

• Linux Virtual Server Administration — Provides information on configuring high-performance
systems and services with the Linux Virtual Server (LVS).

• Red Hat Cluster Suite Release Notes — Provides information about the current release of Red Hat
Cluster Suite.

Red Hat Cluster Suite documentation and other Red Hat documents are available in HTML and PDF
versions online at the following location:

http://www.redhat.com/docs

4. Document Conventions

This manual uses several conventions to highlight certain words and phrases and draw attention to
specific pieces of information.

In PDF and paper editions, this manual uses typefaces drawn from the Liberation Fonts1 set. The
Liberation Fonts set is also used in HTML editions if the set is installed on your system. If not,
alternative but equivalent typefaces are displayed. Note: Red Hat Enterprise Linux 5 and later includes
the Liberation Fonts set by default.

4.1. Typographic Conventions

Four typographic conventions are used to call attention to specific words and phrases. These
conventions, and the circumstances they apply to, are as follows.

Mono-spaced Bold

Used to highlight system input, including shell commands, file names and paths. Also used to highlight
key caps and key-combinations. For example:

To see the contents of the file my_next_bestselling_novel in your current
working directory, enter the cat my_next_bestselling_novel command at the
shell prompt and press Enter to execute the command.

The above includes a file name, a shell command and a key cap, all presented in Mono-spaced Bold
and all distinguishable thanks to context.

Key-combinations can be distinguished from key caps by the hyphen connecting each part of a key­combination. For example:

Press Enter to execute the command.

Press Ctrl+Alt+F1 to switch to the first virtual terminal. Press Ctrl+Alt+F7 to
return to your X-Windows session.

The first sentence highlights the particular key cap to press. The second highlights two sets of three
key caps, each set pressed simultaneously.

If source code is discussed, class names, methods, functions, variable names and returned values
mentioned within a paragraph will be presented as above, in Mono-spaced Bold. For example:

1

https://fedorahosted.org/liberation-fonts/

Typographic Conventions

ix

File-related classes include filesystem for file systems, file for files, and dir for
directories. Each class has its own associated set of permissions.

Proportional Bold

This denotes words or phrases encountered on a system, including application names; dialogue
box text; labelled buttons; check-box and radio button labels; menu titles and sub-menu titles. For
example:

Choose System > Preferences > Mouse from the main menu bar to launch Mouse

Preferences. In the Buttons tab, click the Left-handed mouse check box and click
Close to switch the primary mouse button from the left to the right (making the mouse

suitable for use in the left hand).

To insert a special character into a gedit file, choose Applications > Accessories

> Character Map from the main menu bar. Next, choose Search > Find… from the
Character Map menu bar, type the name of the character in the Search field and

click Next. The character you sought will be highlighted in the Character Table.
Double-click this highlighted character to place it in the Text to copy field and then
click the Copy button. Now switch back to your document and choose Edit > Paste
from the gedit menu bar.

The above text includes application names; system-wide menu names and items; application-specific
menu names; and buttons and text found within a GUI interface, all presented in Proportional Bold and
all distinguishable by context.

Note the > shorthand used to indicate traversal through a menu and its sub-menus. This is to avoid
the difficult-to-follow 'Select Mouse from the Preferences sub-menu in the System menu of the main
menu bar' approach.

Mono-spaced Bold Italic or Proportional Bold Italic

Whether Mono-spaced Bold or Proportional Bold, the addition of Italics indicates replaceable or
variable text. Italics denotes text you do not input literally or displayed text that changes depending on
circumstance. For example:

To connect to a remote machine using ssh, type ssh [e-mail] at
a shell prompt. If the remote machine is example.com and your username on that
machine is john, type ssh [e-mail].

The mount -o remount file-system command remounts the named file
system. For example, to remount the /home file system, the command is mount -o
remount /home.

To see the version of a currently installed package, use the rpm -q package
command. It will return a result as follows: package-version-release.

Note the words in bold italics above — username, domain.name, file-system, package, version and
release. Each word is a placeholder, either for text you enter when issuing a command or for text
displayed by the system.

Aside from standard usage for presenting the title of a work, italics denotes the first use of a new and
important term. For example:

When the Apache HTTP Server accepts requests, it dispatches child processes
or threads to handle them. This group of child processes or threads is known as

Introduction

x

a server-pool. Under Apache HTTP Server 2.0, the responsibility for creating and
maintaining these server-pools has been abstracted to a group of modules called
Multi-Processing Modules (MPMs). Unlike other modules, only one module from the
MPM group can be loaded by the Apache HTTP Server.

4.2. Pull-quote Conventions

Two, commonly multi-line, data types are set off visually from the surrounding text.

Output sent to a terminal is set in Mono-spaced Roman and presented thus:

books Desktop documentation drafts mss photos stuff svn
books_tests Desktop1 downloads images notes scripts svgs

Source-code listings are also set in Mono-spaced Roman but are presented and highlighted as
follows:

package org.jboss.book.jca.ex1;

import javax.naming.InitialContext;

public class ExClient
{
public static void main(String args[])
throws Exception
{
InitialContext iniCtx = new InitialContext();
Object ref = iniCtx.lookup("EchoBean");
EchoHome home = (EchoHome) ref;
Echo echo = home.create();

System.out.println("Created Echo");

System.out.println("Echo.echo('Hello') = " + echo.echo("Hello"));
}

}

4.3. Notes and Warnings

Finally, we use three visual styles to draw attention to information that might otherwise be overlooked.

Note

A note is a tip or shortcut or alternative approach to the task at hand. Ignoring a note
should have no negative consequences, but you might miss out on a trick that makes your
life easier.

Feedback

xi

Important

Important boxes detail things that are easily missed: configuration changes that only
apply to the current session, or services that need restarting before an update will apply.
Ignoring Important boxes won't cause data loss but may cause irritation and frustration.

Warning

A Warning should not be ignored. Ignoring warnings will most likely cause data loss.

5. Feedback

If you spot a typo, or if you have thought of a way to make this manual better, we would love to
hear from you. Please submit a report in Bugzilla (http://bugzilla.redhat.com/bugzilla/) against the
component rh-cs.

Be sure to mention the manual's identifier:

rh-clvm(EN)-4.8 (2009-05-14T12:46)

By mentioning this manual's identifier, we know exactly which version of the guide you have.

If you have a suggestion for improving the documentation, try to be as specific as possible. If you have
found an error, please include the section number and some of the surrounding text so we can find it
easily.

xii

Chapter 1.

1

The LVM Logical Volume Manager

This chapter provides a high-level overview of the components of the Logical Volume Manager (LVM).

1.1. Logical Volumes

Volume management creates a layer of abstraction over physical storage, allowing you to create
logical storage volumes. This provides much greater flexibility in a number of ways than using physical
storage directly.

A logical volume provides storage virtualization. With a logical volume, you are not restricted to
physical disk sizes. In addition, the hardware storage configuration is hidden from the software so it
can be resized and moved without stopping applications or unmounting file systems. This can reduce
operational costs.

Logical volumes provide the following advantages over using physical storage directly:

• Flexible capacity

When using logical volumes, file systems can extend across multiple disks, since you can aggregate
disks and partitions into a single logical volume.

• Resizeable storage pools

You can extend logical volumes or reduce logical volumes in size with simple software commands,
without reformatting and repartitioning the underlying disk devices.

• Online data relocation

To deploy newer, faster, or more resilient storage subsystems, you can move data while your
system is active. Data can be rearranged on disks while the disks are in use. For example, you can
empty a hot-swappable disk before removing it.

• Convenient device naming

Logical storage volumes can be managed in user-defined groups, which you can name according to
your convenience.

• Disk striping

You can create a logical volume that stripes data across two or more disks. This can dramatically
increase throughput.

• Mirroring volumes

Logical volumes provide a convenient way to configure a mirror for your data.

• Volume Snapshots

Using logical volumes, you can take device snapshots for consistent backups or to test the effect of
changes without affecting the real data.

The implementation of these features in LVM is described in the remainder of this document.

Chapter 1. The LVM Logical Volume Manager

2

1.2. LVM Architecture Overview

For the RHEL 4 release of the Linux operating system, the original LVM1 logical volume manager
was replaced by LVM2, which has a more generic kernel framework than LVM1. LVM2 provides the
following improvements over LVM1:

• flexible capacity

• more efficient metadata storage

• better recovery format

• new ASCII metadata format

• atomic changes to metadata

• redundant copies of metadata

LVM2 is backwards compatible with LVM1, with the exception of snapshot and cluster support. You
can convert a volume group from LVM1 format to LVM2 format with the vgconvert command. For
information on converting LVM metadata format, see the vgconvert(8) man page.

The underlying physical storage unit of an LVM logical volume is a block device such as a partition or
whole disk. This device is initialized as an LVM physical volume (PV).

To create an LVM logical volume, the physical volumes are combined into a volume group (VG). This
creates a pool of disk space out of which LVM logical volumes (LVs) can be allocated. This process
is analogous to the way in which disks are divided into partitions. A logical volume is used by file
systems and applications (such as databases).

Figure 1.1, “LVM Logical Volume Components” shows the components of a simple LVM logical

volume:

Figure 1.1. LVM Logical Volume Components

For detailed information on the components of an LVM logical volume, see Chapter 2, LVM

Components.

Running LVM in a Cluster

3

1.3. Running LVM in a Cluster

The Clustered Logical Volume Manager (CLVM) is a set of clustering extensions to LVM. These
extensions allow a cluster of computers to manage shared storage (for example, on a SAN) using
LVM.

The clmvd daemon is the key clustering extension to LVM. The clvmd daemon runs in each cluster
computer and distributes LVM metadata updates in a cluster, presenting each cluster computer with
the same view of the logical volumes.

Figure 1.2, “CLVM Overview” shows a CLVM overview in a Red Hat cluster.

Figure 1.2. CLVM Overview

Logical volumes created with CLVM on shared storage are visible to all computers that have access to
the shared storage.

CLVM allows a user to configure logical volumes on shared storage by locking access to physical
storage while a logical volume is being configured. CLVM uses the locking services provided by the
high availability symmetric infrastructure.

Note

Shared storage for use in Red Hat Cluster Suite requires that you be running the cluster
logical volume manager daemon (clvmd) or the High Availability Logical Volume
Management agents (HA-LVM). If you are not able to use either the clvmd daemon or
HA-LVM for operational reasons or because you do not have the correct entitlements, you

Chapter 1. The LVM Logical Volume Manager

4

must not use single-instance LVM on the shared disk as this may result in data corruption.
If you have any concerns please contact your Red Hat service representative.

Note

CLVM requires changes to the lvm.conf file for cluster-wide locking. For information on
configuring the lvm.conf file to support CLVM, see Section 3.1, “Creating LVM Volumes

in a Cluster”.

You configure LVM volumes for use in a cluster with the standard set of LVM commands or the LVM
graphical user interface, as described in Chapter 4, LVM Administration with CLI Commands and

Chapter 7, LVM Administration with the LVM GUI.

For information on installing LVM in a Red Hat Cluster, see Configuring and Managing a Red Hat
Cluster.

1.4. Document Overview

This remainder of this document includes the following chapters:

• Chapter 2, LVM Components describes the components that make up an LVM logical volume.

• Chapter 3, LVM Administration Overview provides an overview of the basic steps you perform to
configure LVM logical volumes, whether you are using the LVM Command Line Interface (CLI)
commands or the LVM Graphical User Interface (GUI).

• Chapter 4, LVM Administration with CLI Commands summarizes the individual administrative tasks
you can perform with the LVM CLI commands to create and maintain logical volumes.

• Chapter 5, LVM Configuration Examples provides a variety of LVM configuration examples.

• Chapter 6, LVM Troubleshooting provide instructions for troubleshooting a variety of LVM issues.

• Chapter 7, LVM Administration with the LVM GUI summarizes the operating of the LVM GUI.

• Appendix A, The Device Mapper describes the Device Mapper that LVM uses to map logical and
physical volumes.

• Appendix B, The LVM Configuration Files describes the LVM configuration files.

• Appendix C, LVM Object Tags describes LVM object tags and host tags.

• Appendix D, LVM Volume Group Metadata describes LVM volume group metadata, and includes a
sample copy of metadata for an LVM volume group.

Chapter 2.

5

LVM Components

This chapter describes the components of an LVM Logical volume.

2.1. Physical Volumes

The underlying physical storage unit of an LVM logical volume is a block device such as a partition or
whole disk. To use the device for an LVM logical volume the device must be initialized as a physical
volume (PV). Initializing a block device as a physical volume places a label near the start of the
device.

By default, the LVM label is placed in the second 512-byte sector. You can overwrite this default by
placing the label on any of the first 4 sectors. This allows LVM volumes to co-exist with other users of
these sectors, if necessary.

An LVM label provides correct identification and device ordering for a physical device, since devices
can come up in any order when the system is booted. An LVM label remains persistent across reboots
and throughout a cluster.

The LVM label identifies the device as an LVM physical volume. It contains a random unique identifier
(the UUID) for the physical volume. It also stores the size of the block device in bytes, and it records
where the LVM metadata will be stored on the device.

The LVM metadata contains the configuration details of the LVM volume groups on your system.
By default, an identical copy of the metadata is maintained in every metadata area in every physical
volume within the volume group. LVM metadata is small and stored as ASCII.

Currently LVM allows you to store 0, 1 or 2 identical copies of its metadata on each physical volume.
The default is 1 copy. Once you configure the number of metadata copies on the physical volume,
you cannot change that number at a later time. The first copy is stored at the start of the device,
shortly after the label. If there is a second copy, it is placed at the end of the device. If you accidentally
overwrite the area at the beginning of your disk by writing to a different disk than you intend, a second
copy of the metadata at the end of the device will allow you to recover the metadata.

For detailed information about the LVM metadata and changing the metadata parameters, see

Appendix D, LVM Volume Group Metadata.

2.1.1. LVM Physical Volume Layout

Figure 2.1, “Physical Volume layout” shows the layout of an LVM physical volume. The LVM label is

on the second sector, followed by the metadata area, followed by the usable space on the device.

Note

In the Linux kernel (and throughout this document), sectors are considered to be 512
bytes in size.

Chapter 2. LVM Components

6

Figure 2.1. Physical Volume layout

2.1.2. Multiple Partitions on a Disk

LVM allows you to create physical volumes out of disk partitions. It is generally recommended that you
create a single partition that covers the whole disk to label as an LVM physical volume for the following
reasons:

• Administrative convenience

It is easier to keep track of the hardware in a system if each real disk only appears once. This
becomes particularly true if a disk fails. In addition, multiple physical volumes on a single disk may
cause a kernel warning about unknown partition types at boot-up.

• Striping performance

LVM can not tell that two physical volumes are on the same physical disk. If you create a striped
logical volume when two physical volumes are on the same physical disk, the stripes could be on
different partitions on the same disk. This would result in a decrease in performance rather than an
increase.

Although it it is not recommended, there may be specific circumstances when you will need to
divide a disk into separate LVM physical volumes. For example, on a system with few disks it may
be necessary to move data around partitions when you are migrating an existing system to LVM
volumes. Additionally, if you have a very large disk and want to have more than one volume group for
administrative purposes then it is necessary to partition the disk. If you do have a disk with more than
one partition and both of those partitions are in the same volume group, take care to specify which
partitions are to be included in a logical volume when creating striped volumes.

2.2. Volume Groups

Physical volumes are combined into volume groups (VGs). This creates a pool of disk space out of
which logical volumes can be allocated.

Within a volume group, the disk space available for allocation is divided into units of a fixed-size
called extents. An extent is the smallest unit of space that can be allocated, Within a physical volume,
extents are referred to as physical extents.

LVM Logical Volumes

7

A logical volume is allocated into logical extents of the same size as the physical extents. The extent
size is thus the same for all logical volumes in the volume group. The volume group maps the logical
extents to physical extents.

2.3. LVM Logical Volumes

In LVM, a volume group is divided up into logical volumes. There are three types of LVM logical
volumes: linear volumes, striped volumes, and mirrored volumes. These are described in the following
sections.

2.3.1. Linear Volumes

A linear volume aggregates multiple physical volumes into one logical volume. For example, if you
have two 60GB disks, you can create a 120GB logical volume. The physical storage is concatenated.

Creating a linear volume assigns a range of physical extents to an area of a logical volume in order.
For example, as shown in Figure 2.2, “Extent Mapping” logical extents 1 to 99 could map to one
physical volume and logical extents 100 to 198 could map to a second physical volume. From the
point of view of the application, there is one device that is 198 extents in size.

Figure 2.2. Extent Mapping

The physical volumes that make up a logical volume do not have to be the same size. Figure 2.3,

“Linear Volume with Unequal Physical Volumes” shows volume group VG1 with a physical extent size

Chapter 2. LVM Components

8

of 4MB. This volume group includes 2 physical volumes named PV1 and PV2. The physical volumes
are divided into 4MB units, since that is the extent size. In this example, PV1 is 100 extents in size
(400MB) and PV2 is 200 extents in size (800MB). You can create a linear volume any size between
1 and 300 extents (4MB to 1200MB). In this example, the linear volume named LV1 is 300 extents in
size.

Figure 2.3. Linear Volume with Unequal Physical Volumes

You can configure more than one linear logical volume of whatever size you desire from the pool
of physical extents. Figure 2.4, “Multiple Logical Volumes” shows the same volume group as in

Figure 2.3, “Linear Volume with Unequal Physical Volumes”, but in this case two logical volumes have

been carved out of the volume group: LV1, which is 250 extents in size (1000MB) and LV2 which is 50
extents in size (200MB).

Figure 2.4. Multiple Logical Volumes

2.3.2. Striped Logical Volumes

When you write data to an LVM logical volume, the file system lays the data out across the underlying
physical volumes. You can control the way the data is written to the physical volumes by creating a

Mirrored Logical Volumes

9

striped logical volume. For large sequential reads and writes, this can improve the efficiency of the
data I/O.

Striping enhances performance by writing data to a predetermined number of physical volumes in
round-round fashion. With striping, I/O can be done in parallel. In some situations, this can result in
near-linear performance gain for each additional physical volume in the stripe.

The following illustration shows data being striped across three physical volumes. In this figure:

• the first stripe of data is written to PV1

• the second stripe of data is written to PV2

• the third stripe of data is written to PV3

• the fourth stripe of data is written to PV1

In a striped logical volume, the size of the stripe cannnot exceed the size of an extent.

Figure 2.5. Striping Data Across Three PVs

Striped logical volumes can be extended by concatenating another set of devices onto the end of the
first set. In order extend a striped logical volume, however, there must be enough free space on the
underlying physical volumes that make up the volume group to support the stripe. For example, if you
have a two-way stripe that uses up an entire volume group, adding a single physical volume to the
volume group will not enable you to extend the stripe. Instead, you must add at least two physical
volumes to the volume group. For more information on extending a striped volume, see Section 4.4.9,

“Extending a Striped Volume”.

2.3.3. Mirrored Logical Volumes

A mirror maintains identical copies of data on different devices. When data is written to one device, it
is written to a second device as well, mirroring the data. This provides protection for device failures.
When one leg of a mirror fails, the logical volume becomes a linear volume and can still be accessed.

Chapter 2. LVM Components

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LVM supports mirrored volumes. When you create a mirrored logical volume, LVM ensures that data
written to an underlying physical volume is mirrored onto a separate physical volume. With LVM, you
can create mirrored logical volumes with multiple mirrors.

An LVM mirror divides the device being copied into regions that are typically 512KB in size. LVM
maintains a small log which it uses to keep track of which regions are in sync with the mirror or
mirrors. This log can be kept on disk, which will keep it persistent across reboots, or it can be
maintained in memory.

Figure 2.6, “Mirrored Logical Volume” shows a mirrored logical volume with one mirror. In this

configuration, the log is maintained on disk.

Figure 2.6. Mirrored Logical Volume

For information on creating and modifying mirrors, see Section 4.4.1.3, “Creating Mirrored Volumes”.

2.3.4. Snapshot Volumes

The LVM snapshot feature provides the ability to create virtual images of a device at a particular
instant without causing a service interruption. When a change is made to the original device (the
origin) after a snapshot is taken, the snapshot feature makes a copy of the changed data area as it
was prior to the change so that it can reconstruct the state of the device.

Note

LVM snapshots are not supported across the nodes in a cluster.

Because a snapshot copies only the data areas that change after the snapshot is created, the
snapshot feature requires a minimal amount of storage. For example, with a rarely updated origin, 3-5
% of the origin's capacity is sufficient to maintain the snapshot.

Snapshot Volumes

11

Note

Snapshot copies of a file system are virtual copies, not actual media backup for a file
system. Snapshots do not provide a substitute for a backup procedure.

The size of the snapshot governs the amount of space set aside for storing the changes to the origin
volume. For example, if you made a snapshot and then completely overwrote the origin the snapshot
would have to be at least as big as the origin volume to hold the changes. You need to dimension a
snapshot according to the expected level of change. So for example a short-lived snapshot of a read­mostly volume, such as /usr, would need less space than a long-lived snapshot of a volume that
sees a greater number of writes, such as /home.

If a snapshot runs full, the snapshot becomes invalid, since it can no longer track changes on the
origin volumed. You should regularly monitor the size of the snapshot. Snapshots are fully resizeable,
however, so if you have the storage capacity you can increase the size of the snapshot volume to
prevent it from getting dropped. Conversely, if you find that the snapshot volume is larger than you
need, you can reduce the size of the volume to free up space that is needed by other logical volumes.

When you create a snapshot file system, full read and write access to the origin stays possible. If
a chunk on a snapshot is changed, that chunk is marked and never gets copied from the original
volume.

There are several uses for the snapshot feature:

• Most typically, a snapshot is taken when you need to perform a backup on a logical volume without
halting the live system that is continuously updating the data.

• You can execute the fsck command on a snapshot file system to check the file system integrity
and determine whether the original file system requires file system repair.

• Because the snapshot is read/write, you can test applications against production data by taking a
snapshot and running tests against the snapshot, leaving the real data untouched.

• You can create volumes for use with the Xen virtual machine monitor. You can use the snapshot
feature to create a disk image, snapshot it, and modify the snapshot for a particular domU instance.
You can then create another snapshot and modify it for another domU instance. Since the only
storage used is chunks that were changed on the origin or snapshot, the majority of the volume is
shared.

12

Chapter 3.

13

LVM Administration Overview

This chapter provides an overview of the administrative procedures you use to configure LVM logical
volumes. This chapter is intended to provide a general understanding of the steps involved. For
specific step-by-step examples of common LVM configuration procedures, see Chapter 5, LVM

Configuration Examples.

For descriptions of the CLI commands you can use to perform LVM administration, see Chapter 4,

LVM Administration with CLI Commands. Alternately, you can use the LVM GUI, which is described in
Chapter 7, LVM Administration with the LVM GUI.

3.1. Creating LVM Volumes in a Cluster

In order to enable the LVM volumes you are creating in a cluster, the cluster infrastructure must be
running and the cluster must be quorate. Creating clustered logical volumes also requires changes
to the lvm.conf file for cluster-wide locking. Other than this setup, creating LVM logical volumes in
a clustered environment is identical to creating LVM logical volumes on a single node. There is no
difference in the LVM commands themselves, or in the LVM GUI interface.

In order to enable cluster-wide locking, you can run the lvmconf command, as follows:

# /usr/sbin/lvmconf --enable-cluster

Running the lvmconf command modifies the lvm.conf file to specify the appropriate locking type for
clustered volumes.

Note

Shared storage for use in Red Hat Cluster Suite requires that you be running the cluster
logical volume manager daemon (clvmd) or the High Availability Logical Volume
Management agents (HA-LVM). If you are not able to use either the clvmd daemon or
HA-LVM for operational reasons or because you do not have the correct entitlements, you
must not use single-instance LVM on the shared disk as this may result in data corruption.
If you have any concerns please contact your Red Hat service representative.

For information on how to set up the cluster infrastructure, see Configuring and Managing a Red Hat
Cluster.

For an example of creating a mirrored logical volume in a cluster, see Section 5.5, “Creating a

Mirrored LVM Logical Volume in a Cluster”.

3.2. Logical Volume Creation Overview

The following is a summary of the steps to perform to create an LVM logical volume.

1. Initialize the partitions you will use for the LVM volume as physical volumes (this labels them).

2. Create a volume group.

3. Create a logical volume.

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14

After creating the logical volume you can create and mount the file system. The examples in this
document use GFS file systems.

1. Create a GFS file system on the logical volume with the gfs_mkfs command.

2. Create a new mount point with the mkdir command. In a clustered system, create the mount

point on all nodes in the cluster.

3. Mount the file system. You may want to add a line to the fstab file for each node in the system.

Alternately, you can create and mount the GFS file system with the LVM GUI.

Creating the LVM volume is machine independent, since the storage area for LVM setup information
is on the physical volumes and not the machine where the volume was created. Servers that use the
storage have local copies, but can recreate that from what is on the physical volumes. You can attach
physical volumes to a different server if the LVM versions are compatible.

3.3. Growing a File System on a Logical Volume

To grow a file system on a logical volume, perform the following steps:

1. Make a new physical volume.

2. Extend the volume group that contains the logical volume with the file system you are growing to
include the new physical volume.

3. Extend the logical volume to include the new physical volume.

4. Grow the file system.

If you have sufficient unallocated space in the volume group, you can use that space to extend the
logical volume instead of performing steps 1 and 2.

3.4. Logical Volume Backup

Metadata backups and archives are automatically created on every volume group and logical volume
configuration change unless disabled in the lvm.conf file. By default, the metadata backup is stored
in the /etc/lvm/backup file and the metadata archives are stored in the /etc/lvm/archive file.
How long the the metadata archives stored in the /etc/lvm/archive file are kept and how many
archive files are kept is determined by parameters you can set in the lvm.conf file. A daily system
backup should include the contents of the /etc/lvm directory in the backup.

Note that a metadata backup does not back up the user and system data contained in the logical
volumes.

You can manually back up the metadata to the /etc/lvm/backup file with the vgcfgbackup
command. You can restore metadata with the vgcfgrestore command. The vgcfgbackup and
vgcfgrestore commands are described in Section 4.3.11, “Backing Up Volume Group Metadata”.

3.5. Logging

All message output passes through a logging module with independent choices of logging levels for:

• standard output/error

Logging

15

• syslog

• log file

• external log function

The logging levels are set in the /etc/lvm/lvm.conf file, which is described in Appendix B, The

LVM Configuration Files.

16

Chapter 4.

17

LVM Administration with CLI
Commands

This chapter summarizes the individual administrative tasks you can perform with the LVM Command
Line Interface (CLI) commands to create and maintain logical volumes.

Note

If you are creating or modifying an LVM volume for a clustered environment, you must
ensure that you are running the clvmd daemon. For information, see see Section 3.1,

“Creating LVM Volumes in a Cluster”.

4.1. Using CLI Commands

There are several general features of all LVM CLI commands.

When sizes are required in a command line argument, units can always be specified explicitly. If you
do not specify a unit, then a default is assumed, usually KB or MB. LVM CLI commands do not accept
fractions.

When specifying units in a command line argument, LVM is case-insensitive; specifying M or m is
equivalent, for example, and powers of 2 (multiples of 1024) are used. However, when specifying
the --units argument in a command, lower-case indicates that units are in multiples of 1024 while
upper-case indicates that units are in multiples of 1000.

Where commands take volume group or logical volume names as arguments, the full path name
is optional. A logical volume called lvol0 in a volume group called vg0 can be specified as vg0/
lvol0. Where a list of volume groups is required but is left empty, a list of all volume groups will be
substituted. Where a list of logical volumes is required but a volume group is given, a list of all the
logical volumes in that volume group will be substituted. For example, the lvdisplay vg0 command
will display all the logical volumes in volume group vg0.

All LVM commands accept a -v argument, which can be entered multiple times to increase the output
verbosity. For example, the following examples shows the default output of the lvcreate command.

# lvcreate -L 50MB new_vg
Rounding up size to full physical extent 52.00 MB
Logical volume "lvol0" created

The following command shows the output of the lvcreate command with the -v argument.

# lvcreate -v -L 50MB new_vg
Finding volume group "new_vg"
Rounding up size to full physical extent 52.00 MB
Archiving volume group "new_vg" metadata (seqno 4).
Creating logical volume lvol0
Creating volume group backup "/etc/lvm/backup/new_vg" (seqno 5).

Chapter 4. LVM Administration with CLI Commands

18

Found volume group "new_vg"
Creating new_vg-lvol0
Loading new_vg-lvol0 table
Resuming new_vg-lvol0 (253:2)
Clearing start of logical volume "lvol0"
Creating volume group backup "/etc/lvm/backup/new_vg" (seqno 5).
Logical volume "lvol0" created

You could also have used the -vv, -vvv or the -vvvv argument to display increasingly more details
about the command execution. The -vvvv argument provides the maximum amount of information at
this time. The following example shows only the first few lines of output for the lvcreate command
with the -vvvv argument specified.

# lvcreate -vvvv -L 50MB new_vg
#lvmcmdline.c:913 Processing: lvcreate -vvvv -L 50MB new_vg
#lvmcmdline.c:916 O_DIRECT will be used
#config/config.c:864 Setting global/locking_type to 1
#locking/locking.c:138 File-based locking selected.
#config/config.c:841 Setting global/locking_dir to /var/lock/lvm
#activate/activate.c:358 Getting target version for linear
#ioctl/libdm-iface.c:1569 dm version OF [16384]
#ioctl/libdm-iface.c:1569 dm versions OF [16384]
#activate/activate.c:358 Getting target version for striped
#ioctl/libdm-iface.c:1569 dm versions OF [16384]
#config/config.c:864 Setting activation/mirror_region_size to 512
...

You can display help for any of the LVM CLI commands with the --help argument of the command.

commandname --help

To display the man page for a command, execute the man command:

man commandname

The man lvm command provides general online information about LVM.

All LVM objects are referenced internally by a UUID, which is assigned when you create the object.
This can be useful in a situation where you remove a physical volume called /dev/sdf which is part
of a volume group and, when you plug it back in, you find that it is now /dev/sdk. LVM will still find
the physical volume because it identifies the physical volume by its UUID and not its device name. For
information on specifying the UUID of a physical volume when creating a physical volume, see see

Section 6.4, “Recovering Physical Volume Metadata”.

4.2. Physical Volume Administration

This section describes the commands that perform the various aspects of physical volume
administration.