Chapter 2. GFS Overview

The Red Hat GFS file system is a native file system that interfaces directly with the Linux kernel file system interface (VFS layer). When implemented as a cluster file system, GFS employs distributed metadata and multiple journals. Red Hat supports the use of GFS file systems only as implemented in Red Hat Cluster Suite.

Note

Although a GFS file system can be implemented in a standalone system or as part of a cluster configuration, for the Red Hat Enterprise Linux 5.5 release and later Red Hat does not support the use of GFS as a single-node file system. Red Hat does support a number of high-performance single node file systems which are optimized for single node and thus have generally lower overhead than a cluster filesystem. Red Hat recommends using these file systems in preference to GFS in cases where only a single node needs to mount the file system.
Red Hat will continue to support single-node GFS file systems for existing customers.

Note

Red Hat does not support using GFS for cluster file system deployments greater than 16 nodes.
GFS is based on a 64-bit architecture, which can theoretically accommodate an 8 EB file system. However, the current supported maximum size of a GFS file system for 64-bit hardware is 100 TB. The current supported maximum size of a GFS file system for 32-bit hardware is 16 TB. If your system requires larger GFS file systems, contact your Red Hat service representative.
When determining the size of your file system, you should consider your recovery needs. Running the gfs_fsck command on a very large file system can take a long time and consume a large amount of memory. Additionally, in the event of a disk or disk-subsytem failure, recovery time is limited by the speed of your backup media. For information on the amount of memory the gfs_fsck command requires, see Section 4.14, “Repairing a File System”.
When configured in a Red Hat Cluster Suite, Red Hat GFS nodes can be configured and managed with Red Hat Cluster Suite configuration and management tools. Red Hat GFS then provides data sharing among GFS nodes in a Red Hat cluster, with a single, consistent view of the file system name space across the GFS nodes. This allows processes on different nodes to share GFS files in the same way that processes on the same node can share files on a local file system, with no discernible difference. For information about Red Hat Cluster Suite refer to Configuring and Managing a Red Hat Cluster.
While a GFS file system may be used outside of LVM, Red Hat supports only GFS file systems that are created on a CLVM logical volume. CLVM is a cluster-wide implementation of LVM, enabled by the CLVM daemon clvmd, which manages LVM logical volumes in a Red Hat Cluster Suite cluster. The daemon makes it possible to use LVM2 to manage logical volumes across a cluster, allowing all nodes in the cluster to share the logical volumes. For information on the LVM volume manager, see Logical Volume Manager Administration

Note

When you configure a GFS file system as a cluster file system, you must ensure that all nodes in the cluster have access to the shared file system. Asymmetric cluster configurations in which some nodes have access to the file system and others do not are not supported.
This chapter provides some basic, abbreviated information as background to help you understand GFS. It contains the following sections:

2.1. New and Changed Features

This section lists new and changed features included with the initial release of Red Hat Enterprise Linux 5.
  • GULM (Grand Unified Lock Manager) is not supported in Red Hat Enterprise Linux 5. If your GFS file systems use the GULM lock manager, you must convert the file systems to use the DLM lock manager. This is a two-part process.
    • While running Red Hat Enterprise Linux 4, convert your GFS file systems to use the DLM lock manager.
    • Upgrade your operating system to Red Hat Enterprise Linux 5, converting the lock manager to DLM when you do.
    For information on upgrading to Red Hat Enterprise Linux 5 and converting GFS file systems to use the DLM lock manager, see Configuring and Managing a Red Hat Cluster.
  • Documentation for Red Hat Cluster Suite for Red Hat Enterprise Linux 5 has been expanded and reorganized. For information on the available documents, see Section 1.2, “Related Documentation”.