Red Hat Enterprise Linux 4

Global Network Block Device

Using GNBD with Red Hat Global File System

Edition 1.0

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Abstract

This book provides an overview on using Global Network Block Device (GNBD) with Red Hat GFS for Red Hat Enterprise Linux 4.

Introduction

1. About This Guide

This book describes how to use Global Network Block Device (GNDB) with Global File System (GFS), including information about device-mapper multipath, GNDB driver and command usage, and running GFS on a GNBD server node.

2. 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.

3. Software Versions

Table 1. Software Versions

Software	Description
RHEL4	refers to RHEL4 and higher
GFS	refers to GFS 6.1 and higher

4. 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).
LVM Administrator's Guide: Configuration and Administration — Provides a description of the Logical Volume Manager (LVM), including information on running LVM in a clustered environment.
Using Device-Mapper Multipath — Provides information about using the Device-Mapper Multipath feature of Red Hat Enterprise Linux.
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

5. 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 Fonts 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.

5.1. 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 keycaps 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 keycap, all presented in mono-spaced bold and all distinguishable thanks to context.

Key combinations can be distinguished from keycaps by the hyphen connecting each part of a key combination. For example:

Press Enter to execute the command.
Press Ctrl+Alt+F2 to switch to the first virtual terminal. Press Ctrl+Alt+F1 to return to your X-Windows session.

The first paragraph highlights the particular keycap to press. The second highlights two key combinations (each a set of three keycaps with 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:

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; dialog box text; labeled 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.

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 username@domain.name at a shell prompt. If the remote machine is example.com and your username on that machine is john, type ssh john@example.com.
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:

Publican is a DocBook publishing system.

5.2. Pull-quote Conventions

Terminal output and source code listings 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 add syntax highlighting 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"));
   }
}

5.3. Notes and Warnings

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

Note

Notes are tips, shortcuts or alternative approaches 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.

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 a box labeled 'Important' will not cause data loss but may cause irritation and frustration.

Warning

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

6. 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-gfs(EN)-4.8 (2009-05-15T15:10)

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.

Chapter 1. Using GNBD with Red Hat GFS

GNBD (Global Network Block Device) provides block-level storage access over an Ethernet LAN. GNBD components run as a client in a GFS node and as a server in a GNBD server node. A GNBD server node exports block-level storage from its local storage (either directly attached storage or SAN storage) to a GFS node.

Table 1.1, “GNBD Software Subsystem Components” summarizes the GNBD software subsystems components.

Table 1.1. GNBD Software Subsystem Components

Software Subsystem	Components	Description
GNBD	`gnbd.ko`	Kernel module that implements the GNBD device driver on clients.
	`gnbd_export`	Command to create, export and manage GNBDs on a GNBD server.
	`gnbd_import`	Command to import and manage GNBDs on a GNBD client.
	`gnbd_serv`	A server daemon that allows a node to export local storage over the network.

You can configure GNBD servers to work with device-mapper multipath. GNBD with device-mapper multipath allows you to configure multiple GNBD server nodes to provide redundant paths to the storage devices. The GNBD servers, in turn, present multiple storage paths to GFS nodes via redundant GNBDs. When using GNBD with device-mapper multipath, if a GNBD server node becomes unavailable, another GNBD server node can provide GFS nodes with access to storage devices.

This document how to use GNBD with Red Hat GFS and consists of the following chapters:

Chapter 2, Considerations for Using GNBD with Device-Mapper Multipath, which describes some of the issues you should take into account when configuring multipathed GNBD server nodes
Chapter 3, GNBD Driver and Command Usage, which describes the restrictions that apply when you are running GFS on a GNBD server node
Chapter 4, Running GFS on a GNBD Server Node, which describes the user commands that configure GNBD

Chapter 2. Considerations for Using GNBD with Device-Mapper Multipath

2.1. Linux Page Caching
2.2. Fencing GNBD Server Nodes

GNBD with device-mapper multipath allows you to configure multiple GNBD server nodes (nodes that export GNBDs to GFS nodes) to provide redundant paths to the storage devices. The GNBD server nodes, in turn, present multiple storage paths to GFS nodes via redundant GNBDs. When using GNBD with device-mapper multipath, if a GNBD server node becomes unavailable, another GNBD server node can provide GFS nodes with access to storage devices.

If you are using GNBD with device-mapper multipath, you need to take the following into consideration:

Linux page caching, as desribed in Section 2.1, “Linux Page Caching”.
Fencing GNBD server nodes, as described in Section 2.2, “Fencing GNBD Server Nodes”.
GNBD device names; export names for GNBD devices must be unique. Additionally, you must specify the -u or -U when using the gnbd_export command. Exporting GNBD devices is described in Chapter 3, GNBD Driver and Command Usage.

2.1. Linux Page Caching

For GNBD with device-mapper multipath, do not specify Linux page caching (the -c option of the gnbd_export command). All GNBDs that are part of a logical volume must run with caching disabled. Data corruption occurs if the GNBDs are run with caching enabled. Refer to Section 3.1, “Exporting a GNBD from a Server” for more information about using the gnbd_export command for GNBD with device-mapper multipath.

2.2. Fencing GNBD Server Nodes

GNBD server nodes must be fenced using a fencing method that physically removes the nodes from the network. To physically remove a GNBD server node, you can use any fencing device: except the following: fence_brocade fence agent, fence_vixel fence agent, fence_mcdata fence agent, fence_sanbox2 fence agent, fence_scsi fence agent. In addition, you cannot use the GNBD fencing device (fence_gnbd fence agent) to fence a GNBD server node. For information about configuring fencing for GNBD server nodes, refer to the Global File System manual.

The Global Network Block Device (GNBD) driver allows a node to export its local storage as a GNBD over a network so that other nodes on the network can share the storage. Client nodes importing the GNBD use it like any other block device. Importing a GNBD on multiple clients forms a shared storage configuration through which GFS can be used.

The GNBD driver is implemented through the following components.

gnbd_serv — Implements the GNBD server. It is a user-space daemon that allows a node to export local storage over a network.
gnbd.ko — Implements the GNBD device driver on GNBD clients (nodes using GNBD devices).

Two user commands are available to configure GNBD:

gnbd_export (for servers) — User program for creating, exporting, and managing GNBDs on a GNBD server.
gnbd_import (for clients) — User program for importing and managing GNBDs on a GNBD client.

3.1. Exporting a GNBD from a Server

The gnbd_serv daemon must be running on a node before it can export storage as a GNBD. You can start the gnbd_serv daemon running gnbd_serv as follows:

#gnbd_serv
gnbd_serv: startup succeeded

Once local storage has been identified to be exported, the gnbd_export command is used to export it.

Warning

When you configure GNBD servers with device-mapper multipath, you must not use page caching. All GNBDs that are part of a logical volume must run with caching disabled. By default, the gnbd_export command exports with caching turned off.

Note

A server should not import the GNBDs to use them as a client would. If a server exports the devices uncached, the underlying devices may also be used by gfs.

Usage

gnbd_export -d pathname -e gnbdname [-c][-u][-U

pathname: Specifies a storage device to export.
gnbdname: Specifies an arbitrary name selected for the GNBD. It is used as the device name on GNBD clients. This name must be unique among all GNBDs exported in a network.
-o: Export the device as read-only.
-c: Enable caching. Reads from the exported GNBD and takes advantage of the Linux page cache.
By default, the gnbd_export command does not enable caching.
Warning
When you configure GNBD servers with device-mapper multipath, do not specify the -c option, as this lead sto data corruption. All GNBDs that are part of a logical volume must run with caching disabled.
Note
If you have been using GFS 5.2 or earlier and do not want to change your GNBD setup you should specify the -c option. Before GFS Release 5.2.1, Linux caching was enabled by default for gnbd_export. If the -c option is not specified, GNBD runs with a noticeable performance decrease. Also, if the -c option is not specified, the exported GNBD runs in timeout mode, using the default timeout value (the -t option). For more information about the gnbd_export command and its options, refer to the gnbd_export man page.
-u uid: Manually sets the Universal Identifier for an exported device. This option is used with -e. The UID is used by device-mapper multipath to determine which devices belong in a multipath map. A device must have a UID to be multipathed. However, for most SCSI devices the default Get UID command, /usr/sbin/gnbd_get_uid, will return an appropriate value.
Note
The UID refers to the device being exported, not the GNBD itself. The UIDs of two GNBD devices should be equal, only if they are exporting the same underlying device. This means that both GNBD servers are connected to the same physical device.
Warning
This option should only be used for exporting shared storage devices, when the -U command option does not work. This should almost never happen for SCSI devices. If two GNBD devices are not exporting the same underlying device, but are given the same UID, data corruption will occur.
-U Command: Gets the UID command. The UID command is a command the gnbd_export command will run to get a Universal Identifier for the exported device. The UID is necessary to use device-mapper multipath with GNBD. The command must use the full path of any executeable that you wish to run. A command can contain the %M, %m or %n escape sequences. %M will be expanded to the major number of the exported device, %m will be expaned to the minor number of the exported device, and %n will be expanded to the sysfs name for the device. If no command is given, GNBD will use the default command /usr/sbin/gnbd_get_uid. This command will work for most SCSI devices.

Examples

This example is for a GNBD server configured with GNBD multipath. It exports device /dev/sdc2 as GNBD gamma. Cache is disabled by default.

gnbd_export -d /dev/sdc2 -e gamma -U

This example is for a GNBD server not configured with GNBD multipath. It exports device /dev/sdb2 as GNBD delta with cache enabled.

gnbd_export -d /dev/sdb1 -e delta -c

This example exports device /dev/sdb2 as GNBD delta with cache enabled.

gnbd_export -d /dev/sdb2 -e delta -c

3.2. Importing a GNBD on a Client

The gnbd.ko kernel module must be loaded on a node before it can import GNBDs. When GNBDs are imported, device nodes are created for them in /dev/gnbd/ with the name assigned when they were exported.

Usage

gnbd_import -i Server

Server: Specifies a GNBD server by hostname or IP address from which to import GNBDs. All GNBDs exported from the server are imported on the client running this command.

Example

This example imports all GNBDs from the server named nodeA.

gnbd_import -i nodeA

Chapter 4. Running GFS on a GNBD Server Node

You can run GFS on a GNBD server node, with some restrictions. In addition, running GFS on a GNBD server node reduces performance. The following restrictions apply when running GFS on a GNBD server node.

Important

When running GFS on a GNBD server node you must follow the restrictions listed; otherwise, the GNBD server node will fail.

A GNBD server node must have local access to all storage devices needed to mount a GFS file system. The GNBD server node must not import (gnbd_import command) other GNBD devices to run the file system.
The GNBD server must export all the GNBDs in uncached mode, and it must export the raw devices, not logical volume devices.
GFS must be run on top of a logical volume device, not raw devices.

Note

You may need to increase the timeout period on the exported GNBDs to accommodate reduced performance. The need to increase the timeout period depends on the quality of the hardware.

Revision History

Revision History

Revision 1.0

Wed Apr 01 2009

Index

D

device-mapper multipath, Considerations for Using GNBD with Device-Mapper Multipath

fencing GNBD server nodes, Fencing GNBD Server Nodes
Linux page caching, Linux Page Caching

driver and command usage, GNBD Driver and Command Usage

exporting from a server, Exporting a GNBD from a Server
importing on a client, Importing a GNBD on a Client

E

exporting from a server daemon, Exporting a GNBD from a Server

F

feedback, Feedback
fencing GNBD server nodes, Fencing GNBD Server Nodes

G

GFS, using on a GNBD server node, Running GFS on a GNBD Server Node
GNBD, using with Red Hat GFS, Using GNBD with Red Hat GFS
gnbd.ko module, GNBD Driver and Command Usage, Importing a GNBD on a Client
gnbd_export command , GNBD Driver and Command Usage, Usage
gnbd_import command , GNBD Driver and Command Usage, Usage
gnbd_serv daemon, GNBD Driver and Command Usage, Exporting a GNBD from a Server

I

importing on a client module, Importing a GNBD on a Client

L

Linux page caching, Linux Page Caching

S

software subsystem components, Using GNBD with Red Hat GFS