1.4.2. Additional Differences Between GFS and GFS2

This section summarizes the additional differences in GFS and GFS2 administration that are not described in Section 1.4.1, “GFS2 Command Names”.

Context-Dependent Path Names

GFS2 file systems do not provide support for context-dependent path names, which allow you to create symbolic links that point to variable destination files or directories. For this functionality in GFS2, you can use the bind option of the mount command. For information on bind mounts and context-dependent pathnames in GFS2, see Section 4.12, “Bind Mounts and Context-Dependent Path Names”.

gfs2.ko Module

The kernel module that implements the GFS file system is gfs.ko. The kernel module that implements the GFS2 file system is gfs2.ko.

Enabling Quota Enforcement in GFS2

In GFS2 file systems, quota enforcement is disabled by default and must be explicitly enabled. For information on enabling and disabling quota enforcement, see Section 4.5, “GFS2 Quota Management”.

Data Journaling

GFS2 file systems support the use of the chattr command to set and clear the j flag on a file or directory. Setting the +j flag on a file enables data journaling on that file. Setting the +j flag on a directory means "inherit jdata", which indicates that all files and directories subsequently created in that directory are journaled. Using the chattr command is the preferred way to enable and disable data journaling on a file.

Adding Journals Dynamically

In GFS file systems, journals are embedded metadata that exists outside of the file system, making it necessary to extend the size of the logical volume that contains the file system before adding journals. In GFS2 file systems, journals are plain (though hidden) files. This means that for GFS2 file systems, journals can be dynamically added as additional servers mount a file system, as long as space remains on the file system for the additional journals. For information on adding journals to a GFS2 file system, see Section 4.7, “Adding Journals to a File System”.

atime_quantum parameter removed

The GFS2 file system does not support the atime_quantum tunable parameter, which can be used by the GFS file system to specify how often atime updates occur. In its place GFS2 supports the relatime and noatime mount options. The relatime mount option is recommended to achieve similar behavior to setting the atime_quantum parameter in GFS.

The data= option of the mount command

When mounting GFS2 file systems, you can specify the data=ordered or data=writeback option of the mount. When data=ordered is set, the user data modified by a transaction is flushed to the disk before the transaction is committed to disk. This should prevent the user from seeing uninitialized blocks in a file after a crash. When data=writeback is set, the user data is written to the disk at any time after it is dirtied. This does not provide the same consistency guarantee as ordered mode, but it should be slightly faster for some workloads. The default is ordered mode.

The gfs2_tool command

The gfs2_tool command supports a different set of options for GFS2 than the gfs_tool command supports for GFS:
  • The gfs2_tool command supports a journals parameter that prints out information about the currently configured journals, including how many journals the file system contains.
  • The gfs2_tool command does not support the counters flag, which the gfs_tool command uses to display GFS statistics.
  • The gfs2_tool command does not support the inherit_jdata flag. To flag a directory as "inherit jdata", you can set the jdata flag on the directory or you can use the chattr command to set the +j flag on the directory. Using the chattr command is the preferred way to enable and disable data journaling on a file.

Note

As of the Red Hat Enterprise Linux 6.2 release, GFS2 supports the tunegfs2 command, which replaces some of the features of the gfs2_tool command. For further information, refer to the tunegfs2(8) man page. The settune and gettune functions of the gfs2_tool command have been replaced by command line options of the mount command, which allows them to be set by means of the fstab file when required.

The gfs2_edit command

The gfs2_edit command supports a different set of options for GFS2 than the gfs_edit command supports for GFS. For information on the specific options each version of the command supports, see the gfs2_edit and gfs_edit man pages.