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GFS2: Add a document explaining GFS2's uevents
This will be essential reading for anybody who wants to understand how GFS2 interacts with the userland gfs_controld, and the details of recovery. Signed-off-by: Steven Whitehouse <swhiteho@redhat.com> Signed-off-by: Bob Peterson <rpeterso@redhat.com>
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uevents and GFS2 | ||
================== | ||
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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). | ||
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A list of GFS2 uevents | ||
----------------------- | ||
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1. ADD | ||
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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. | ||
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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. | ||
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2. ONLINE | ||
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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. | ||
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3. CHANGE | ||
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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. | ||
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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. | ||
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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. | ||
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4. OFFLINE | ||
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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. | ||
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5. REMOVE | ||
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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 preceeded by at least an ADD uevent for the same fileystem, | ||
and unlike the other uevents is generated automatically by the kernel's | ||
kobject subsystem. | ||
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Information common to all GFS2 uevents (uevent environment variables) | ||
---------------------------------------------------------------------- | ||
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1. LOCKTABLE= | ||
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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. | ||
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2. LOCKPROTO= | ||
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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. | ||
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3. JOURNALID= | ||
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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. | ||
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4. UUID= | ||
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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. | ||
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