[GFS2] Add documentation for GFS2

Signed-off-by: David Teigland <[e-mail]>
Signed-off-by: Steve Whitehouse <[e-mail]>

Documentation/filesystems/gfs2.txt

···
+Global File System
+------------------
+
+http://sources.redhat.com/cluster/
+
+GFS is a cluster file system. It allows a cluster of computers to
+simultaneously use a block device that is shared between them (with FC,
+iSCSI, NBD, etc).  GFS reads and writes to the block device like a local
+file system, but also uses a lock module to allow the computers coordinate
+their I/O so file system consistency is maintained.  One of the nifty
+features of GFS is perfect consistency -- changes made to the file system
+on one machine show up immediately on all other machines in the cluster.
+
+GFS uses interchangable inter-node locking mechanisms.  Different lock
+modules can plug into GFS and each file system selects the appropriate
+lock module at mount time.  Lock modules include:
+
+  lock_nolock -- allows gfs to be used as a local file system
+
+  lock_dlm -- uses a distributed lock manager (dlm) for inter-node locking
+  The dlm is found at linux/fs/dlm/
+
+In addition to interfacing with an external locking manager, a gfs lock
+module is responsible for interacting with external cluster management
+systems.  Lock_dlm depends on user space cluster management systems found
+at the URL above.
+
+To use gfs as a local file system, no external clustering systems are
+needed, simply:
+
+  $ gfs2_mkfs -p lock_nolock -j 1 /dev/block_device
+  $ mount -t gfs2 /dev/block_device /dir
+
+GFS2 is not on-disk compatible with previous versions of GFS.
+
+The following man pages can be found at the URL above:
+  gfs2_mkfs	to make a filesystem
+  gfs2_fsck	to repair a filesystem
+  gfs2_grow	to expand a filesystem online
+  gfs2_jadd	to add journals to a filesystem online
+  gfs2_tool	to manipulate, examine and tune a filesystem
+  gfs2_quota	to examine and change quota values in a filesystem
+  mount.gfs2	to find mount options
+