The dmfilemapd daemon monitors groups of dmstats(8) regions that
correspond to the extents of a file, adding and removing regions
to reflect the changing state of the file on-disk.
The daemon is normally launched automatically by the dmstatscreate command, but can be run manually, either to create a new
daemon where one did not previously exist, or to change the
options previously used, by killing the existing daemon and
starting a new one.
Specify the file descriptor number for the file to be
monitored. The file descriptor must reference a regular
file, open for reading, in a local file system that
supports the FIEMAP ioctl, and that returns data
describing the physical location of extents.
The process that executes dmfilemapd is responsible for
opening the file descriptor that is handed to the daemon.
The dmstats group identifier of the group that dmfilemapd
should update. The group must exist and it should
correspond to a set of regions created by a previous
The absolute path to the file being monitored, at the time
that it was opened. The use of abs_path by the daemon
differs, depending on the filemap following mode in use;
see MODES and the mode option for more information.
The filemap monitoring mode the daemon. Use either inode
(DM_FILEMAP_FOLLOW_INODE), or path
(DM_FILEMAP_FOLLOW_PATH), to enable follow-inode or
follow-path mode respectively.
If set to 1, disable forking and allow the daemon to run
in the foreground.
Control daemon logging. If set to zero, the daemon will
close all stdio streams and run silently. If verbose is a
number between 1 and 3, stdio will be retained and the
daemon will log messages to stdout and stderr that match
the specified verbosity level.
The file map monitoring daemon can monitor files in two distinct
ways: the mode affects the behaviour of the daemon when a file
under monitoring is renamed or unlinked, and the conditions which
cause the daemon to terminate.
In both modes, the daemon will always shut down when the group
being monitored is deleted.
The daemon follows the inode of the file, as it was at the time
the daemon started. The file descriptor referencing the file is
kept open at all times, and the daemon will exit when it detects
that the file has been unlinked and it is the last holder of a
reference to the file.
This mode is useful if the file is expected to be renamed, or
moved within the file system, while it is being monitored.
The daemon follows the path that was given on the daemon command
line. The file descriptor referencing the file is re-opened on
each iteration of the daemon, and the daemon will exit if no file
exists at this location (a tolerance is allowed so that a brief
delay between removal and replacement is permitted).
This mode is useful if the file is updated by unlinking the
original and placing a new file at the same path.
The daemon attempts to maintain good synchronisation between the
file extents and the regions contained in the group, however,
since the daemon can only react to new allocations once they have
been written, there are inevitably some IO events that cannot be
counted when a file is growing, particularly if the file is being
extended by a single thread writing beyond EOF (for example, the
There is a further loss of events in that there is currently no
way to atomically resize a dmstats region and preserve its
current counter values. This affects files when they grow by
extending the final extent, rather than allocating a new extent:
any events that had accumulated in the region between any prior
operation and the resize are lost.
File mapping is currently most effective in cases where the
majority of IO does not trigger extent allocation. Future updates
may address these limitations when kernel support is available.
Normally the daemon is started automatically by the dmstatscreate or update_filemap commands but it can be run manually for
debugging or testing purposes.
Start the daemon in the background, in follow-path mode
# dmfilemapd 3 0 /srv/images/vm.img path 0 0 3</srv/images/vm.img
Start the daemon in follow-inode mode, disable forking and enable
# dmfilemapd 3 0 /var/tmp/data inode 1 3 3< /var/tmp/data
Starting dmfilemapd with fd=3, group_id=0 mode=inode, path=/var/tmp/data
dm version [ opencount flush ]  (*1)
dm info (253:0) [ opencount flush ]  (*1)
dm message (253:0) [ opencount flush ] @stats_list dmstats  (*1)
Read alias 'data' from aux_data
Found group_id 0: alias="data"
dm_stats_walk_init: initialised flags to 4000000000000
starting stats walk with GROUP
exiting _filemap_monitor_get_events() with deleted=0, check=0
Waiting for check interval