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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | MODES | LIMITATIONS | EXAMPLES | AUTHORS | SEE ALSO | COLOPHON |
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DMFILEMAPD(8) MAINTENANCE COMMANDS DMFILEMAPD(8)
dmfilemapd — device-mapper filemap monitoring daemon
dmfilemapd file_descriptor group_id abs_path inode|path
[foreground [verbose]]
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.
file_descriptor
Specify the file descriptor number for the file to be moni‐
tored. 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 corre‐
spond to a set of regions created by a previous filemap op‐
eration.
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 dae‐
mon 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 un‐
der 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 be‐
ing 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 origi‐
nal 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
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 major‐
ity 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 dmstats create
or update_filemap commands but it can be run manually for debug‐
ging 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
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
Bryn M. Reeves <bmr@redhat.com>
dmstats(8)
LVM2 resource page: ⟨https://www.sourceware.org/lvm2⟩
Device-mapper resource page: ⟨http://sources.redhat.com/dm⟩
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 2025-02-02. (At that time,
the date of the most recent commit that was found in the
repository was 2025-01-31.) 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
man-pages@man7.org
Linux Dec 17 2016 DMFILEMAPD(8)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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