uevents and GFS2
                               ==================
  
  During the lifetime of a GFS2 mount, a number of uevents are generated.
  This document explains what the events are and what they are used
  for (by gfs_controld in gfs2-utils).
  
  A list of GFS2 uevents
  -----------------------
  
  1. ADD
  
  The ADD event occurs at mount time. It will always be the first
  uevent generated by the newly created filesystem. If the mount
  is successful, an ONLINE uevent will follow.  If it is not successful
  then a REMOVE uevent will follow.
  
  The ADD uevent has two environment variables: SPECTATOR=[0|1]
  and RDONLY=[0|1] that specify the spectator status (a read-only mount
  with no journal assigned), and read-only (with journal assigned) status
  of the filesystem respectively.
  
  2. ONLINE
  
  The ONLINE uevent is generated after a successful mount or remount. It
  has the same environment variables as the ADD uevent. The ONLINE
  uevent, along with the two environment variables for spectator and
  RDONLY are a relatively recent addition (2.6.32-rc+) and will not
  be generated by older kernels.
  
  3. CHANGE
  
  The CHANGE uevent is used in two places. One is when reporting the
  successful mount of the filesystem by the first node (FIRSTMOUNT=Done).
  This is used as a signal by gfs_controld that it is then ok for other
  nodes in the cluster to mount the filesystem.
  
  The other CHANGE uevent is used to inform of the completion
  of journal recovery for one of the filesystems journals. It has
  two environment variables, JID= which specifies the journal id which
  has just been recovered, and RECOVERY=[Done|Failed] to indicate the
  success (or otherwise) of the operation. These uevents are generated
  for every journal recovered, whether it is during the initial mount
  process or as the result of gfs_controld requesting a specific journal
  recovery via the /sys/fs/gfs2/<fsname>/lock_module/recovery file.
  
  Because the CHANGE uevent was used (in early versions of gfs_controld)
  without checking the environment variables to discover the state, we
  cannot add any more functions to it without running the risk of
  someone using an older version of the user tools and breaking their
  cluster. For this reason the ONLINE uevent was used when adding a new
  uevent for a successful mount or remount.
  
  4. OFFLINE
  
  The OFFLINE uevent is only generated due to filesystem errors and is used
  as part of the "withdraw" mechanism. Currently this doesn't give any
  information about what the error is, which is something that needs to
  be fixed.
  
  5. REMOVE
  
  The REMOVE uevent is generated at the end of an unsuccessful mount
  or at the end of a umount of the filesystem. All REMOVE uevents will

  have been preceded by at least an ADD uevent for the same filesystem,

  and unlike the other uevents is generated automatically by the kernel's
  kobject subsystem.
  
  
  Information common to all GFS2 uevents (uevent environment variables)
  ----------------------------------------------------------------------
  
  1. LOCKTABLE=
  
  The LOCKTABLE is a string, as supplied on the mount command
  line (locktable=) or via fstab. It is used as a filesystem label
  as well as providing the information for a lock_dlm mount to be
  able to join the cluster.
  
  2. LOCKPROTO=
  
  The LOCKPROTO is a string, and its value depends on what is set
  on the mount command line, or via fstab. It will be either
  lock_nolock or lock_dlm. In the future other lock managers
  may be supported.
  
  3. JOURNALID=
  
  If a journal is in use by the filesystem (journals are not
  assigned for spectator mounts) then this will give the
  numeric journal id in all GFS2 uevents.
  
  4. UUID=
  
  With recent versions of gfs2-utils, mkfs.gfs2 writes a UUID
  into the filesystem superblock. If it exists, this will
  be included in every uevent relating to the filesystem.