1DMFILEMAPD(8) MAINTENANCE COMMANDS DMFILEMAPD(8)
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6 dmfilemapd — device-mapper filemap monitoring daemon
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9 dmfilemapd file_descriptor group_id abs_path inode|path [foreground
10 [verbose]]
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13 The dmfilemapd daemon monitors groups of dmstats(8) regions that corre‐
14 spond to the extents of a file, adding and removing regions to reflect
15 the changing state of the file on-disk.
16 The daemon is normally launched automatically by the dmstats create
18 command, but can be run manually, either to create a new daemon where
19 one did not previously exist, or to change the options previously used,
20 by killing the existing daemon and starting a new one.
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23 file_descriptor
24 Specify the file descriptor number for the file to be monitored.
25 The file descriptor must reference a regular file, open for
26 reading, in a local file system that supports the FIEMAP ioctl,
27 and that returns data describing the physical location of ex‐
28 tents.
29 The process that executes dmfilemapd is responsible for opening
31 the file descriptor that is handed to the daemon.
32 group_id
34 The dmstats group identifier of the group that dmfilemapd should
35 update. The group must exist and it should correspond to a set
36 of regions created by a previous filemap operation.
37 abs_path
39 The absolute path to the file being monitored, at the time that
40 it was opened. The use of abs_path by the daemon differs, de‐
41 pending on the filemap following mode in use; see MODES and the
42 mode option for more information.
43 inode|path
45 The filemap monitoring mode the daemon. Use either inode
46 (DM_FILEMAP_FOLLOW_INODE), or path (DM_FILEMAP_FOLLOW_PATH), to
47 enable follow-inode or follow-path mode respectively.
48 [foreground]
50 If set to 1, disable forking and allow the daemon to run in the
51 foreground.
52 [verbose]
54 Control daemon logging. If set to zero, the daemon will close
55 all stdio streams and run silently. If verbose is a number be‐
56 tween 1 and 3, stdio will be retained and the daemon will log
57 messages to stdout and stderr that match the specified verbosity
58 level.
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61 The file map monitoring daemon can monitor files in two distinct ways:
62 the mode affects the behaviour of the daemon when a file under monitor‐
63 ing is renamed or unlinked, and the conditions which cause the daemon
64 to terminate.
65 In both modes, the daemon will always shut down when the group being
67 monitored is deleted.
68 Follow inode
70 The daemon follows the inode of the file, as it was at the time the
72 daemon started. The file descriptor referencing the file is kept open
73 at all times, and the daemon will exit when it detects that the file
74 has been unlinked and it is the last holder of a reference to the file.
75 This mode is useful if the file is expected to be renamed, or moved
77 within the file system, while it is being monitored.
78 Follow path
80 The daemon follows the path that was given on the daemon command line.
82 The file descriptor referencing the file is re-opened on each iteration
83 of the daemon, and the daemon will exit if no file exists at this loca‐
84 tion (a tolerance is allowed so that a brief delay between removal and
85 replacement is permitted).
86 This mode is useful if the file is updated by unlinking the original
88 and placing a new file at the same path.
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91 The daemon attempts to maintain good synchronisation between the file
92 extents and the regions contained in the group, however, since the dae‐
93 mon can only react to new allocations once they have been written,
94 there are inevitably some IO events that cannot be counted when a file
95 is growing, particularly if the file is being extended by a single
96 thread writing beyond EOF (for example, the dd program).
97 There is a further loss of events in that there is currently no way to
99 atomically resize a dmstats region and preserve its current counter
100 values. This affects files when they grow by extending the final ex‐
101 tent, rather than allocating a new extent: any events that had accumu‐
102 lated in the region between any prior operation and the resize are
103 lost.
104 File mapping is currently most effective in cases where the majority of
106 IO does not trigger extent allocation. Future updates may address these
107 limitations when kernel support is available.
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110 Normally the daemon is started automatically by the dmstats create or
111 update_filemap commands but it can be run manually for debugging or
112 testing purposes.
113 Start the daemon in the background, in follow-path mode
115 # dmfilemapd 3 0 /srv/images/vm.img path 0 0 3< /srv/images/vm.img
116 Start the daemon in follow-inode mode, disable forking and enable ver‐
118 bose logging
119 # dmfilemapd 3 0 /var/tmp/data inode 1 3 3< /var/tmp/data
120 Starting dmfilemapd with fd=3, group_id=0 mode=inode, path=/var/tmp/data
121 dm version [ opencount flush ] [16384] (*1)
122 dm info (253:0) [ opencount flush ] [16384] (*1)
123 dm message (253:0) [ opencount flush ] @stats_list dmstats [16384] (*1)
124 Read alias 'data' from aux_data
125 Found group_id 0: alias="data"
126 dm_stats_walk_init: initialised flags to 4000000000000
127 starting stats walk with GROUP
128 exiting _filemap_monitor_get_events() with deleted=0, check=0
129 Waiting for check interval
130
133 Bryn M. Reeves <bmr@redhat.com>
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136 dmstats(8)
137 LVM2 resource page: ⟨https://www.sourceware.org/lvm2⟩
139 Device-mapper resource page: ⟨http://sources.redhat.com/dm⟩
140Linux Dec 17 2016 DMFILEMAPD(8)