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DMFILEMAPD(8)                MAINTENANCE COMMANDS                DMFILEMAPD(8)
NAME
       dmfilemapd -- device-mapper filemap monitoring daemon
SYNOPSIS
       dmfilemapd file_descriptor group_id abs_path inode|path [foreground
       [verbose]]
DESCRIPTION
       The dmfilemapd daemon monitors groups of dmstats(8) regions that corre-
       spond  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  dmstats  create
       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.
OPTIONS
       file_descriptor
              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  ex-
              tents.
              The  process that executes dmfilemapd is responsible for opening
              the file descriptor that is handed to the daemon.
       group_id
              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 filemap operation.
       abs_path
              The absolute path to the file being monitored, at the time  that
              it  was  opened.  The use of abs_path by the daemon differs, de-
              pending on the filemap following mode in use; see MODES and  the
              mode option for more information.
       inode|path
              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.
       [foreground]
              If  set to 1, disable forking and allow the daemon to run in the
              foreground.
       [verbose]
              Control daemon logging. If set to zero, the  daemon  will  close
              all  stdio  streams and run silently. If verbose is a number be-
              tween 1 and 3, stdio will be retained and the  daemon  will  log
              messages to stdout and stderr that match the specified verbosity
              level.
MODES
       The file map monitoring daemon can monitor files in two distinct  ways:
       the mode affects the behaviour of the daemon when a file under monitor-
       ing 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.
       Follow inode
       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.
       Follow path
       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 loca-
       tion  (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.
LIMITATIONS
       The  daemon  attempts to maintain good synchronisation between the file
       extents and the regions contained in the group, however, since the dae-
       mon  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 dd program).
       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  ex-
       tent,  rather than allocating a new extent: any events that had accumu-
       lated 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.
EXAMPLES
       Normally  the  daemon is started automatically by the dmstats create 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 ver-
       bose logging
       # 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 ]   [16384] (*1)
       dm info   (253:0) [ opencount flush ]   [16384] (*1)
       dm message   (253:0) [ opencount flush ]  @stats_list dmstats [16384] (*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
AUTHORS
       Bryn M. Reeves <bmr AT redhat.com>
SEE ALSO
       dmstats(8)
       LVM2 resource page: <https://www.sourceware.org/lvm2>;
       Device-mapper resource page: <http://sources.redhat.com/dm>;
Linux                             Dec 17 2016                    DMFILEMAPD(8)