dmfilemapd(8) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | MODES | LIMITATIONS | EXAMPLES | AUTHORS | SEE ALSO | COLOPHON

DMFILEMAPD(8)             MAINTENANCE COMMANDS             DMFILEMAPD(8)

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 cre‐
       ate command, but can be run manually, either to create a new dae‐
       mon  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

       file_descriptor
              Specify the file descriptor number for the file to be mon‐
              itored.  The file descriptor must reference a regular
              file, open for reading, in a local file system that sup‐
              ports 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 corre‐
              spond 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 dif‐
              fers, depending 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_FOL‐
              LOW_PATH), to enable follow-inode or follow-path mode re‐
              spectively.

       [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         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 orig‐
       inal 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 cur‐
       rent counter values. This affects files when they grow by extend‐
       ing the final extent, rather than allocating a new extent: any
       events that had accumulated in the region between any prior oper‐
       ation and the resize are lost.

       File mapping is currently most effective in cases where the ma‐
       jority 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 cre‐
       ate or update_filemap commands but it can be run manually for de‐
       bugging 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/im‐
       ages/vm.img

       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 <[email protected]>

SEE ALSO         top

       dmstats(8)

       LVM2 resource page: ⟨https://www.sourceware.org/lvm2⟩
       Device-mapper resource page: ⟨http://sources.redhat.com/dm

COLOPHON         top

       This page is part of the lvm2 (Logical Volume Manager 2) project.
       Information    about   the   project   can   be   found   at   
       ⟨http://www.sourceware.org/lvm2/⟩.  If you have a bug  report  for
       this  manual  page, see ⟨https://github.com/lvmteam/lvm2/issues⟩.
       This page was obtained from the project's upstream Git repository
       ⟨git://sourceware.org/git/lvm2.git⟩  on  2024-06-14.   (At   that
       time,  the  date  of the most recent commit that was found in the
       repository  was  2024-06-11.)   If  you  discover  any  rendering
       problems  in  this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or  you  have
       corrections  or  improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a  mail  to
       [email protected]

Linux                          Dec 17 2016                 DMFILEMAPD(8)