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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.
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.
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.
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.
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)
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.
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.
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.