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Global File System
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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:

  $ mkfs -t gfs2 -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_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 help mount(8) mount a filesystem
  mkfs.gfs2     to make a filesystem