The Red Hat GFS2 file system is a native file system that interfaces directly with the Linux kernel file system interface (VFS layer). A GFS2 file system can be implemented in a standalone system or as part of a cluster configuration. When implemented as a cluster file system, GFS2 employs distributed metadata and multiple journals.
A GFS2 file system can be created on an LVM logical volume. A logical volume is an aggregation of underlying block devices that appears as a single logical device. For information on the LVM volume manager, see the LVM Administrator's Guide.
GFS2 is based on a 64-bit architecture, which can theoretically accommodate an 8 EB file system. However, the current supported maximum size of a GFS2 file system is 25 TB. If your system requires GFS2 file systems larger than 25 TB, contact your Red Hat service representative.
When determining the size of your file system, you should consider your recovery needs. Running the 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.
When configured in a Red Hat Cluster Suite, Red Hat GFS2 nodes can be configured and managed with Red Hat Cluster Suite configuration and management tools. Red Hat GFS2 then provides data sharing among GFS2 nodes in a Red Hat cluster, with a single, consistent view of the file system name space across the GFS2 nodes. This allows processes on different nodes to share GFS2 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.
LVM logical volumes in a Red Hat Cluster suite are managed with CLVM, which is a cluster-wide implementation of LVM, enabled by the CLVM daemon, clvmd running 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. to any directory on your system. For information on the LVM volume manager, see the LVM Administrator's Guide.
The gfs2.ko kernel module implements the GFS2 file system and is loaded on GFS2 cluster nodes.
This chapter provides some basic, abbreviated information as background to help you understand GFS2. It contains the following sections:
1.1. Before Setting Up GFS2
Before you install and set up GFS2, note the following key characteristics of your GFS2 file systems:
- GFS2 nodes
Determine which nodes in the Red Hat Cluster Suite will mount the GFS2 file systems.
- Number of file systems
Determine how many GFS2 file systems to create initially. (More file systems can be added later.)
- File system name
Determine a unique name for each file system. The name must be unique for all lock_dlm filesystems over the cluster, and for all filesystems (lock_dlm and lock_nolock) on each local node. Each file system name is required in the form of a parameter variable. For example, this book uses file system names mydata1 and mydata2 in some example procedures.
- Journals
Determine the number of journals for your GFS2 file systems. One journal is required for each node that mounts a GFS2 file system. GFS2 allows you to add journals dynamically at a later point as additional servers mount a filesystem. For information on adding journals to a GFS2 file system, see
Section 3.6, “Adding Journals to a File System”.
- GNBD server nodes
If you are using GNBD, determine how many GNBD server nodes are needed. Note the hostname and IP address of each GNBD server node for setting up GNBD clients later. For information on using GNBD with GFS2, see the Using GNBD with Global File System document.
- Storage devices and partitions
Determine the storage devices and partitions to be used for creating logical volumes (via CLVM) in the file systems.
Note
You may see performance problems with GFS2 when many create and delete operations are issued from more than one node in the same directory at the same time. If this causes performance problems in your system, you should localize file creation and deletions by a node to directories specific to that node as much as possible.
