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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] [mode] [fore-
       ground[verbose]]
DESCRIPTION
       The dmfilemapd daemon monitors groups of dmstats  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
              extents.
              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 path by the daemon differs, depending
              on the filemap following mode in use; see  MODES  and  the  mode
              option for more information.

       mode
              The  filemap  monitoring  mode  the  daemon  should  use: either
              "inode" (DM_FILEMAP_FOLLOW_INODE),  or  "path"  (DM_FILEMAP_FOL-
              LOW_PATH),  to  enable  follow-inode or follow-path mode respec-
              tively.
       [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
              between  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
       extent, rather than allocating a new extent: any events that had  accu-
       mulated  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)