Chapter 6. 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.
This chapter describes how to use GNBD with Red Hat GFS and consists of the following sections:
 | Note |
|---|---|
Multipath GNBD is not available with Red Hat GFS 6.1. That is, device mapper multipath (dm-multipath) cannot use GNBD. GNBD without multipath is available. |
6.1. GNBD Driver and Command Usage
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.
6.1.1. 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.
| Note |
|---|---|
A server should not import the GNBDs to use them as a client would. If a server exports the devices uncached, they may also be used by ccsd and gfs. |
6.1.1.1. Usage
gnbd_export -d pathname -e gnbdname [-c] |
- 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.
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.
6.1.1.2. Examples
This example exports device /dev/sdb2 as GNBD delta with cache enabled.
gnbd_export -d /dev/sdb1 -e delta -c |
6.1.2. 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.
6.1.2.1. 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.