linux-libre/Documentation/device-mapper/dm-uevent.txt

The device-mapper uevent code adds the capability to device-mapper to create
and send kobject uevents (uevents).  Previously device-mapper events were only
available through the ioctl interface.  The advantage of the uevents interface
is the event contains environment attributes providing increased context for
the event avoiding the need to query the state of the device-mapper device after
the event is received.

There are two functions currently for device-mapper events.  The first function
listed creates the event and the second function sends the event(s).

void dm_path_uevent(enum dm_uevent_type event_type, struct dm_target *ti,
                    const char *path, unsigned nr_valid_paths)

void dm_send_uevents(struct list_head *events, struct kobject *kobj)


The variables added to the uevent environment are:

Variable Name: DM_TARGET
Uevent Action(s): KOBJ_CHANGE
Type: string
Description:
Value: Name of device-mapper target that generated the event.

Variable Name: DM_ACTION
Uevent Action(s): KOBJ_CHANGE
Type: string
Description:
Value: Device-mapper specific action that caused the uevent action.
	PATH_FAILED - A path has failed.
	PATH_REINSTATED - A path has been reinstated.

Variable Name: DM_SEQNUM
Uevent Action(s): KOBJ_CHANGE
Type: unsigned integer
Description: A sequence number for this specific device-mapper device.
Value: Valid unsigned integer range.

Variable Name: DM_PATH
Uevent Action(s): KOBJ_CHANGE
Type: string
Description: Major and minor number of the path device pertaining to this
event.
Value: Path name in the form of "Major:Minor"

Variable Name: DM_NR_VALID_PATHS
Uevent Action(s): KOBJ_CHANGE
Type: unsigned integer
Description:
Value: Valid unsigned integer range.

Variable Name: DM_NAME
Uevent Action(s): KOBJ_CHANGE
Type: string
Description: Name of the device-mapper device.
Value: Name

Variable Name: DM_UUID
Uevent Action(s): KOBJ_CHANGE
Type: string
Description: UUID of the device-mapper device.
Value: UUID. (Empty string if there isn't one.)

An example of the uevents generated as captured by udevmonitor is shown
below.

1.) Path failure.
UEVENT[1192521009.711215] change@/block/dm-3
ACTION=change
DEVPATH=/block/dm-3
SUBSYSTEM=block
DM_TARGET=multipath
DM_ACTION=PATH_FAILED
DM_SEQNUM=1
DM_PATH=8:32
DM_NR_VALID_PATHS=0
DM_NAME=mpath2
DM_UUID=mpath-35333333000002328
MINOR=3
MAJOR=253
SEQNUM=1130

2.) Path reinstate.
UEVENT[1192521132.989927] change@/block/dm-3
ACTION=change
DEVPATH=/block/dm-3
SUBSYSTEM=block
DM_TARGET=multipath
DM_ACTION=PATH_REINSTATED
DM_SEQNUM=2
DM_PATH=8:32
DM_NR_VALID_PATHS=1
DM_NAME=mpath2
DM_UUID=mpath-35333333000002328
MINOR=3
MAJOR=253
SEQNUM=1131