dmfilemapd(8) — Linux manual page



NAME         top

       dmfilemapd — device-mapper filemap monitoring daemon

SYNOPSIS         top

       dmfilemapd file_descriptor group_id abs_path inode|path
       [foreground [verbose]]

DESCRIPTION         top

       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 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         top

              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
              filemap operation.

              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.

MODES         top

       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.

       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 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.

LIMITATIONS         top

       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
       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 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.

EXAMPLES         top

       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<

       Start the daemon in follow-inode mode, disable forking and enable
       verbose 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         top

       Bryn M. Reeves <>

SEE ALSO         top


       LVM2 resource page: ⟨⟩
       Device-mapper resource page: ⟨

COLOPHON         top

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Linux                          Dec 17 2016                 DMFILEMAPD(8)