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] [mode] [fore‐
10 ground[verbose]]
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13 The dmfilemapd daemon monitors groups of dmstats 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
28 extents.
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 path by the daemon differs, depending
41 on the filemap following mode in use; see MODES and the mode
42 option for more information.
43 mode
46 The filemap monitoring mode the daemon should use: either
47 "inode" (DM_FILEMAP_FOLLOW_INODE), or "path" (DM_FILEMAP_FOL‐
48 LOW_PATH), to enable follow-inode or follow-path mode respec‐
49 tively.
50 [foreground]
52 If set to 1, disable forking and allow the daemon to run in the
53 foreground.
54 [verbose] Control daemon logging. If set to zero, the daemon will close
56 all stdio streams and run silently. If verbose is a number
57 between 1 and 3, stdio will be retained and the daemon will log
58 messages to stdout and stderr that match the specified verbosity
59 level.
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62 The file map monitoring daemon can monitor files in two distinct ways:
63 the mode affects the behaviour of the daemon when a file under monitor‐
64 ing is renamed or unlinked, and the conditions which cause the daemon
65 to terminate.
66 In both modes, the daemon will always shut down when the group being
68 monitored is deleted.
69 Follow inode
72 The daemon follows the inode of the file, as it was at the time the
74 daemon started. The file descriptor referencing the file is kept open
75 at all times, and the daemon will exit when it detects that the file
76 has been unlinked and it is the last holder of a reference to the file.
77 This mode is useful if the file is expected to be renamed, or moved
79 within the file system, while it is being monitored.
80 Follow path
83 The daemon follows the path that was given on the daemon command line.
85 The file descriptor referencing the file is re-opened on each iteration
86 of the daemon, and the daemon will exit if no file exists at this loca‐
87 tion (a tolerance is allowed so that a brief delay between removal and
88 replacement is permitted).
89 This mode is useful if the file is updated by unlinking the original
91 and placing a new file at the same path.
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94 The daemon attempts to maintain good synchronisation between the file
95 extents and the regions contained in the group, however, since the dae‐
96 mon can only react to new allocations once they have been written,
97 there are inevitably some IO events that cannot be counted when a file
98 is growing, particularly if the file is being extended by a single
99 thread writing beyond EOF (for example, the dd program).
100 There is a further loss of events in that there is currently no way to
102 atomically resize a dmstats region and preserve its current counter
103 values. This affects files when they grow by extending the final
104 extent, rather than allocating a new extent: any events that had accu‐
105 mulated in the region between any prior operation and the resize are
106 lost.
107 File mapping is currently most effective in cases where the majority of
109 IO does not trigger extent allocation. Future updates may address these
110 limitations when kernel support is available.
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113 Normally the daemon is started automatically by the dmstats create or
114 update_filemap commands but it can be run manually for debugging or
115 testing purposes.
116 Start the daemon in the background, in follow-path mode
118 # dmfilemapd 3 0 /srv/images/vm.img path 0 0 3< /srv/images/vm.img
119 Start the daemon in follow-inode mode, disable forking and enable ver‐
121 bose logging
122 # dmfilemapd 3 0 /var/tmp/data inode 1 3 3< /var/tmp/data
123 Starting dmfilemapd with fd=3, group_id=0 mode=inode,
124 path=/var/tmp/data
125 dm version [ opencount flush ] [16384] (*1)
126 dm info (253:0) [ opencount flush ] [16384] (*1)
127 dm message (253:0) [ opencount flush ] @stats_list dmstats [16384]
128 (*1)
129 Read alias 'data' from aux_data
130 Found group_id 0: alias="data"
131 dm_stats_walk_init: initialised flags to 4000000000000
132 starting stats walk with GROUP
133 exiting _filemap_monitor_get_events() with deleted=0, check=0
134 Waiting for check interval
135
137 Bryn M. Reeves <bmr@redhat.com>
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140 dmstats(8)
141 LVM2 resource page: https://www.sourceware.org/lvm2/
143 Device-mapper resource page: http://sources.redhat.com/dm/
144Linux Dec 17 2016 DMFILEMAPD(8)