1 # SPDX-License-Identifier: GPL-2.0
8 dss creates hardlink-based snapshots of a given directory on a remote
9 or local host using rsync's link-dest feature.
11 purpose = the dyadic snapshot scheduler
12 synopsis = [global-options...] [--] [<subcommand> [subcommand-options...]]
14 [option general-options-section]
15 summary = General options
18 summary = print help and exit
20 [option detailed-help]
21 summary = print help, including all details, and exit
23 summary = print version and exit
27 summary = use alternative config file (default: ~/.dssrc)
29 arg_info = required_arg
32 Options may be given at the command line or in the configuration
33 file. As usual, if an option is given both at the command line and
34 in the configuration file, the command line option takes precedence.
36 However, there is one exception to this rule: The run subcommand
37 re-reads the configuration file when it receives the HUP signal. In
38 this case the options in the config file override any options that
39 were previously given at the command line. This allows changing the
40 configuration of a running dss process by sending SIGHUP.
44 summary = set loglevel (0-6)
46 arg_info = required_arg
50 Lower values mean more verbose logging.
54 summary = only print what would be done
56 This flag does not make sense for all subcommands. The run subcommand
57 refuses to start if this option was given while the ls subcommand
58 silently ignores the flag.
61 summary = the remote directory to snapshot
63 arg_info = required_arg
66 The directory on the remote host from which snapshots are taken.
67 Of course, the user specified as --remote-user must have read access
70 This option is mandatory for the create and run subcommands: It must
71 be given at the command line or in the config file.
74 summary = where snapshots are stored
76 arg_info = required_arg
79 The destination directory on the local host where snapshots will be
80 written. This must be writable by the user who runs dss.
82 This option is mandatory for all subcommands except kill.
85 summary = abort if destination directory is not a mountpoint
87 This option checks whether a file system is mounted on the directory
88 specified as the argument to --dest-dir. Operation proceeds only
89 if this is the case. Otherwise dss exits unsuccessfully without
90 performing any action. Use this option to prevent snapshot creation
91 if the snapshot file system is not mounted.
93 This option is silently ignored for subcommands which do not depend
94 on the destination directory.
96 [option Rsync-options]
97 summary = Controlling how rsync is run
100 These options are only relevant to the run and the create subcommands.
104 summary = host to take snapshots from
106 arg_info = required_arg
108 default_val = localhost
110 If this option is given and its value differs from the local
111 host, then rsync uses ssh. Make sure there is no password
112 needed for the ssh connection. To achieve that, use public key
113 authentication for ssh and, if needed, set the remote user name
114 by using the --remote-user option.
118 summary = Remote user name (default: current user)
119 arg_info = required_arg
123 Set this if the user that runs dss is different from the user on the
127 summary = run rsync with --checksum occasionally
129 arg_info = required_arg
133 If a file on the backup becomes corrupt in a way that file size
134 and modification time still match the original file, rsync will not
135 consider the file for transfer ("quick check"). Hence the corruption
136 stays on the backup until the file is modified on the source.
137 The --checksum option of rsync disables the quick check and compares
138 the contents of each file, fixing such corruptions. Since computing
139 the checksums adds a significant slowdown due to a lot of disk I/O,
140 the option is not enabled by default.
142 The argument to the --checksum option of dss is a number between 0
143 and 1000, inclusively, which determines the probability of adding
144 --checksum to the rsync options each time a snapshot is created. The
145 default value zero means to never add the option. The value 100 will
146 create every tenth snapshot (on average) using checksums, and the
147 value 1000 will always pass --checksum to rsync.
149 [option rsync-option]
151 summary = further rsync options
153 arg_info = required_arg
157 This option may be given multiple times. The given argument is
158 passed verbatim to the rsync command. Note that in order to use
159 rsync options that require an argument, you have to specify the
160 option and its argument as separate --rsync-options, like this:
162 --rsync-option --exclude --rsync-option /proc
165 summary = Fine tuning the number of snapshots per time unit
168 Snapshot aging is implemented in terms of intervals. There are two
169 command line options related to intervals: the duration u of a unit
170 interval and the number of unit intervals, denoted n below.
172 dss removes snapshots older than n times u and tries to keep 2^(n -
173 k - 1) snapshots in interval k, where the interval number k counts
174 from zero to n - 1, with zero being the most recent unit interval.
176 Hence the oldest snapshot will at most be u * n days old (4 days *
177 5 intervals = 20 days, if default values are used). Moreover, there
178 are at most 2^n - 1 snapshots in total (2^5 - 1 = 31 by default). Note
179 that for this to work out your system must be fast enough to create at
180 least 2^(n - 1) snapshots per unit interval (16 snapshots in 4 days =
181 one snapshot in 6 hours), because this is the number of snapshots in
184 [option unit-interval]
186 summary = the duration of a unit interval
188 arg_info = required_arg
192 Increasing this number instructs dss to create fewer snapshots per
193 time unit while the number of snapshots to keep stays the same.
195 [option num-intervals]
197 summary = the number of unit intervals
199 arg_info = required_arg
203 Increasing this number by one doubles the total number of
207 summary = Commands to be run on certain events
210 All hooks default to "true". That is, the true(1) utility (which
211 always returns with exit code zero) is executed if the hook command
214 [option pre-create-hook]
216 summary = executed before a snapshot is created
218 arg_info = required_arg
222 This command is executed before dss runs rsync to create a new
223 snapshot. If the command returns with a non-zero exit status, no
224 snapshot will be created and the operation is retried later.
226 For example, the command could execute a script that checks whether
227 all snapshot-related file systems are mounted.
229 Another possible application of the pre-create hook is to return
230 non-zero during office hours in order to not slow down the file
231 systems by taking snapshots.
233 [option post-create-hook]
234 summary = executed after a snapshot has been created
236 arg_info = required_arg
240 This is only executed if a snapshot has successfully been created. The
241 full path of the newly created snapshot is passed to the hook as the
242 first argument. The exit code of this hook is ignored.
244 For instance this hook could count the number of files per user
245 and/or compute disk usage patterns to be stored in a database for
248 [option pre-remove-hook]
249 summary = executed before a snapshot is removed
251 arg_info = required_arg
255 The full path to the snapshot which is about to be removed is passed
256 to the command as the first argument. If the command returns with
257 a non-zero exit status, the snapshot is not going to be removed and
258 the operation is retried later.
260 For example, one could execute a script that checks whether the
261 snapshot to be deleted is currently used by another process, e.g. by
262 a tape-based backup system that runs concurrently to dss.
264 Another possible application of this is to record disk-usage
265 patterns before and after snapshot removal.
267 [option post-remove-hook]
268 summary = executed after snapshot removal
270 arg_info = required_arg
274 As for the pre-remove hook, the full path of the removed snapshot is
275 passed to the hook as the first argument. The exit code of this hook
279 summary = executed before the run command exits
281 arg_info = required_arg
285 This hook is only relevant to the run subcommand. It is executed just
286 before dss terminates. The reason for termination is passed as the
289 One possible application for this hook is to send email to the system
290 administrator to let her know that no more snapshots are going to
294 [option disk-space-monitoring]
295 summary = Disk space monitoring
298 The options of this section control the aggressiveness of snapshot
299 removal. That is, they define under which circumstances existing
300 snapshots are removed. These options are only relevant to the run
301 and the prune subcommands.
305 summary = minimal amount of free disk space
306 arg_info = required_arg
311 If disk space on the file system containing the destination
312 directory gets low, the run subcommand suspends the currently
313 running rsync process and starts to remove snapshots in order to
314 free disk space. This option specifies the minimal amount of free
315 disk space. If less than the given number of megabytes is available,
316 snapshots are being deleted. See also the --min_free_percent and the
317 min-free-percent-inodes options below.
319 A value of zero deactivates this check.
321 [option min-free-percent]
323 summary = minimal percentage of free disk space
324 arg_info = required_arg
329 This is like --min-free-mb but the amount of free disk space
330 is specified as a percentage. It is not recommended to set both
331 --min-free-mb and --min-free-percent to zero as this will cause your
332 file system to fill up quickly.
334 [option min-free-percent-inodes]
336 summary = minimal percent of free inodes
337 arg_info = required_arg
342 The minimum amount of free inodes on the file system containing the
343 destination dir. If the percentage of free inodes drops below the
344 given value, snapshot removal kicks in like in case of low disk space.
346 The number of free inodes is determined from the f_ffree field of
347 the statvfs structure. However, some file systems set this field to
348 zero, indicating that the number of inodes is basically unlimited.
349 Moreover it is not possible to reliably detect whether this is the
350 case. Therefore this feature is disabled by default. It's safe to
351 enable it for ext2/ext3/ext4 file systems on linux though.
353 A value of zero (the default) deactivates this check.
355 [option keep-redundant]
357 summary = prune by disk space only
359 By default, redundant and outdated snapshots are removed automatically
360 to keep the number of snapshots in harmony with the configured
361 policy. If this flag is given, dss removes such snapshots only if
362 disk space or number of free inodes becomes low.
364 [option min-complete]
365 summary = minimal number of complete snapshots to keep
366 arg_info = optional_arg
371 This option is only relevant if snapshots must be deleted because
374 dss refuses to remove old snapshots if there are fewer complete
375 snapshots left than the given number. The default value of one
376 guarantees that at least one complete snapshot is available at
379 If only <num> complete snapshots are left, and there is not enough
380 disk space available for another snapshot, the program terminates
381 with a "No space left on device" error.
385 dss supports the subcommands described below. If no subcommand is
386 given, the list of available subcommands is shown and the program
387 terminates successfully without performing any further action.
391 purpose = start creating and pruning snapshots
393 This is the main mode of operation. Snapshots are created in an endless
394 loop as needed and pruned automatically. The loop only terminates on
395 fatal errors or if a terminating signal was received. See also the
400 summary = run as background daemon
402 If this option is given, the dss command detaches from the console
403 and continues to run in the background. It is not possible to let
404 a daemonized process re-attach to the console by editing the config
405 file and sending SIGHUP. However, log output may be redirected to a
406 different file in this way.
412 summary = where to write log output
413 arg_info = required_arg
416 default_val = /dev/null
418 This option is only honored if --daemon is given, in which case
419 log messages go to the given file. Otherwise the option is silently
420 ignored and log output is written to stderr.
422 [option max-rsync-errors]
423 summary = terminate after this many rsync failures
425 arg_info = required_arg
429 If the rsync process exits with a fatal error, dss restarts the command
430 in the hope that the problem is transient and subsequent rsync runs
431 succeed. After the given number of consecutive rsync error exits,
432 however, dss gives up, executes the exit hook and terminates. Set
433 this to zero if dss should exit immediately on the first rsync error.
435 The only non-fatal error is when rsync exits with code 24. This
436 indicates a partial transfer due to vanished source files and happens
437 frequently when snapshotting a directory which is concurrently being
441 purpose = execute rsync once to create a new snapshot
443 This command does not check the amount free disk space. The pre-create
444 and post-create hooks are honored, however.
446 Specify --dry-run to see the rsync command which is executed to create
450 purpose = remove redundant and outdated snapshots
452 A snapshot is considered outdated if its interval number is greater or
453 equal than the specified number of unit intervals. See --unit-interval
454 and --num-intervals above.
456 A snapshot is said to be redundant if the interval it belongs to
457 contains more than the configured number of snapshots.
459 The prune command gets rid of both outdated and redundant snapshots. At
460 most one snapshot is removed per invocation. If --dry-run is given, the
461 subcommand only prints the snapshot that would be removed.
464 purpose = print the list of all snapshots
466 The list contains all existing snapshots, no matter of their state.
467 Incomplete snapshots and snapshots being deleted will also be listed.
470 purpose = send a signal to a running dss process
472 This sends a signal to the dss process that corresponds to the given
473 config file. If --dry-run is given, the PID of the dss process is
474 written to stdout, but no signal is sent.
478 summary = send the given signal rather than SIGTERM
480 arg_info = required_arg
482 default_val = SIGTERM
484 Like for kill(1), alternate signals may be specified in three ways: as
485 a signal number (e.g., 9), the signal name (e.g., KILL), or the signal
486 name prefixed with "SIG" (e.g., SIGKILL). In the latter two forms,
487 the signal name and the prefix are case insensitive, so "sigkill"
490 Sending SIGHUP causes the running dss process to reload its config file.
494 summary = wait until the signalled process has terminated
496 This option is handy for system shutdown scripts which would like
497 to terminate the dss daemon process.
499 Without --wait the dss process which executes the kill subcommand
500 exits right after the kill(2) system call returns. At this point the
501 signalled process might still be alive (even if SIGKILL was sent).
502 If --wait is given, the process waits until the signalled process
503 has terminated or the timeout expires.
505 If --wait is not given, the kill subcommand exits successfully if
506 and only if the signal was sent (i.e., if there exists another dss
507 process to receive the signal). With --wait it exits successfully
508 if, additionally, the signalled process has terminated before the
511 It makes only sense to use the option for signals which terminate dss.
513 [subcommand configtest]
514 purpose = run a configuration file syntax test
516 This command checks the command line options and the configuration
517 file for syntactic correctness. It either reports "Syntax Ok" and
518 exits successfully or prints information about the first syntax error
519 detected and terminates with exit code 1.
523 Written by Andre Noll
525 Copyright (C) 2008 - present Andre Noll
527 License: GNU GPL version 2
529 This is free software: you are free to change and redistribute it.
531 There is NO WARRANTY, to the extent permitted by law.
534 .MT <maan@tuebingen.mpg.de>