Editing GFS2 (section)

== Journaling ==

GFS and GFS2 are both [[journaled file system]]s; and GFS2 supports a similar set of journaling modes as [[ext3]]. In ''data=writeback'' mode, only metadata is journaled. This is the only mode supported by GFS, however it is possible to turn on journaling on individual data-files, but only when they are of zero size. Journaled files in GFS have a number of restrictions placed upon them, such as
no support for the [[mmap]] or sendfile system calls, they also use a different on-disk format from regular files. There is also an "inherit-journal" attribute which when set on a directory causes all files (and sub-directories) created within that directory to have the journal (or inherit-journal, respectively) flag set. This can be used instead of the ''data=journal'' mount option which [[ext3]] supports (and GFS/GFS2 does not).

GFS2 also supports ''data=ordered'' mode which is similar to ''data=writeback'' except that dirty data is synced before each journal flush is completed. This ensures that blocks which have been added to an inode will have their content synced back to disk before the metadata is updated to record the new size and thus prevents uninitialised blocks appearing in a file under node failure conditions. The default journaling mode is ''data=ordered'', to match [[ext3]]'s default.

{{As of | 2010}}, GFS2 does not yet support ''data=journal'' mode, but it does (unlike GFS) use the same on-disk format for both regular and journaled files, and it also supports the same journaled and inherit-journal attributes. GFS2 also relaxes the restrictions on when a file may have its journaled attribute changed to any time that the file is not open (also the same as [[ext3]]).

For performance reasons, each node in GFS and GFS2 has its own journal. In GFS the journals are disk extents, in GFS2 the journals are just regular files. The number of nodes which may mount the filesystem at any one time is limited by the number of available journals.