Editing GFS2 (section)

== Compatibility and the GFS2 meta filesystem ==

GFS2 was designed so that upgrading from GFS would be a simple procedure. To this end, most of the on-disk structure has remained the same as GFS, including the [[Endianness#Big-endian|big-endian]] byte ordering. There are a few differences though:

* GFS2 has a "meta filesystem" through which processes access system files
* GFS2 uses the same on-disk format for journaled files as for regular files
* GFS2 uses regular (system) files for journals, whereas GFS uses special extents
* GFS2 has some other "{{mono|per_node}}" system files
* The layout of the [[inode]] is (very slightly) different
* The layout of indirect blocks differs slightly

The journaling systems of GFS and GFS2 are not compatible with each other. Upgrading is possible by means of a tool ({{mono|gfs2_convert}}) which is run with the filesystem off-line to update the metadata. Some spare blocks in the GFS journals are used to create the (very small) {{mono|per_node}} files required by GFS2 during the update process. Most of the data remains in place.

The GFS2 "meta filesystem" is not a filesystem in its own right, but an alternate [[Root directory|root]] of the main filesystem. Although it behaves like a "normal" filesystem, its contents are the various system files used by GFS2, and normally users do not need to ever look at it. The GFS2 utilities [[Mount (computing)|mount]] and unmount the meta filesystem as required, behind the scenes.