6 dss creates hardlink-based snapshots of a given directory on a remote
7 or local host using rsync's link-dest feature.
9 purpose = the dyadic snapshot scheduler
10 synopsis = [global-options...] [--] [<subcommand> [subcommand-options...]]
12 [option general-options-section]
13 summary = General options
16 summary = print help and exit
18 [option detailed-help]
19 summary = print help, including all details, and exit
21 summary = print version and exit
25 summary = use alternative config file (default: ~/.dssrc)
27 arg_info = required_arg
30 Options may be given at the command line or in the configuration
31 file. As usual, if an option is given both at the command line and
32 in the configuration file, the command line option takes precedence.
34 However, there is one exception to this rule: The run subcommand
35 re-reads the configuration file when it receives the HUP signal. In
36 this case the options in the config file override any options that
37 were previously given at the command line. This allows to change the
38 configuration of a running dss process by sending SIGHUP.
42 summary = set loglevel (0-6)
44 arg_info = required_arg
48 Lower values mean more verbose logging.
52 summary = only print what would be done
54 This flag does not make sense for all subcommands. The run subcommand
55 refuses to start if this option was given while the ls subcommand
56 silently ignores the flag.
59 summary = the remote directory to snapshot
61 arg_info = required_arg
64 The directory on the remote host from which snapshots are taken.
65 Of course, the user specified as --remote-user must have read access
68 This option is mandatory for the create and run subcommands: It must
69 be given at the command line or in the config file.
72 summary = where snapshots are stored
74 arg_info = required_arg
77 The destination directory on the local host where snapshots will be
78 written. This must be writable by the user who runs dss.
80 This option is mandatory for all subcommands except kill.
82 [option Rsync-options]
83 summary = Controlling how rsync is run
86 These options are only relevant to the run and the create subcommands.
90 summary = host to take snapshots from
92 arg_info = required_arg
94 default_val = localhost
96 If this option is given and its value differs from the local
97 host, then rsync uses ssh. Make sure there is no password
98 needed for the ssh connection. To achieve that, use public key
99 authentication for ssh and, if needed, set the remote user name
100 by using the --remote-user option.
104 summary = Remote user name (default: current user)
105 arg_info = required_arg
109 Set this if the user that runs dss is different from the user on the
112 [option rsync-option]
114 summary = further rsync options
116 arg_info = required_arg
120 This option may be given multiple times. The given argument is
121 passed verbatim to the rsync command. Note that in order to use
122 rsync options that require an argument, you have to specify the
123 option and its argument as separate --rsync-options, like this:
125 --rsync-option --exclude --rsync-option /proc
128 summary = Fine tuning the number of snapshots per time unit
131 Snapshot aging is implemented in terms of intervals. There are two
132 command line options related to intervals: the duration u of a unit
133 interval and the number of unit intervals, denoted n below.
135 dss removes snapshots older than n times u and tries to keep 2^(n -
136 k - 1) snapshots in interval k, where the interval number k counts
137 from zero to n - 1, with zero being the most recent unit interval.
139 Hence the oldest snapshot will at most be u * n days old (4 days *
140 5 intervals = 20 days, if default values are used). Moreover, there
141 are at most 2^n - 1 snapshots in total (2^5 - 1 = 31 by default). Note
142 that for this to work out your system must be fast enough to create at
143 least 2^(n - 1) snapshots per unit interval (16 snapshots in 4 days =
144 one snapshot in 6 hours), because this is the number of snapshots in
147 [option unit-interval]
149 summary = the duration of a unit interval
151 arg_info = required_arg
155 Increasing this number instructs dss to create fewer snapshots per
156 time unit while the number of snapshots to keep stays the same.
158 [option num-intervals]
160 summary = the number of unit intervals
162 arg_info = required_arg
166 Increasing this number by one doubles the total number of
170 summary = Commands to be run on certain events
173 All hooks default to "true". That is, the true(1) utility (which
174 always returns with exit code zero) is executed if the hook command
177 [option pre-create-hook]
179 summary = executed before a snapshot is created
181 arg_info = required_arg
185 This command is executed before dss runs rsync to create a new
186 snapshot. If the command returns with a non-zero exit status, no
187 snapshot will be created and the operation is retried later.
189 For example, the command could execute a script that checks whether
190 all snapshot-related file systems are mounted.
192 Another possible application of the pre-create hook is to return
193 non-zero during office hours in order to not slow down the file
194 systems by taking snapshots.
196 [option post-create-hook]
197 summary = executed after a snapshot has been created
199 arg_info = required_arg
203 This is only executed if a snapshot has successfully been created. The
204 full path of the newly created snapshot is passed to the hook as the
205 first argument. The exit code of this hook is ignored.
207 For instance this hook could count the number of files per user
208 and/or compute disk usage patterns to be stored in a database for
211 [option pre-remove-hook]
212 summary = executed before a snapshot is removed
214 arg_info = required_arg
218 The full path to the snapshot which is about to be removed is passed
219 to the command as the first argument. If the command returns with
220 a non-zero exit status, the snapshot is not going to be removed and
221 the operation is retried later.
223 For example, one could execute a script that checks whether the
224 snapshot to be deleted is currently used by another process, e.g. by
225 a tape-based backup system that runs concurrently to dss.
227 Another possible application of this is to record disk-usage
228 patterns before and after snapshot removal.
230 [option post-remove-hook]
231 summary = executed after snapshot removal
233 arg_info = required_arg
237 As for the pre-remove hook, the full path of the removed snapshot is
238 passed to the hook as the first argument. The exit code of this hook
242 summary = executed before the run command exits
244 arg_info = required_arg
248 This hook is only relevant to the run subcommand. It is executed just
249 before dss terminates. The reason for termination is passed as the
252 One possible application for this hook is to send email to the system
253 administrator to let her know that no more snapshots are going to
257 [option disk-space-monitoring]
258 summary = Disk space monitoring
261 The options of this section control the aggressiveness of snapshot
262 removal. That is, they define under which circumstances existing
263 snapshots are removed. These options are only relevant to the run
264 and the prune subcommands.
268 summary = minimal amount of free disk space
269 arg_info = required_arg
274 If disk space on the file system containing the destination
275 directory gets low, the run subcommand suspends the currently
276 running rsync process and starts to remove snapshots in order to
277 free disk space. This option specifies the minimal amount of free
278 disk space. If less than the given number of megabytes is available,
279 snapshots are being deleted. See also the --min_free_percent and the
280 min-free-percent-inodes options below.
282 A value of zero deactivates this check.
284 [option min-free-percent]
286 summary = minimal percentage of free disk space
287 arg_info = required_arg
292 This is like --min-free-mb but allows to specify the amount of
293 free disk space as a percentage. It is not recommended to set both
294 --min-free-mb and --min-free-percent to zero as this will cause your
295 file system to fill up quickly.
297 [option min-free-percent-inodes]
299 summary = minimal percent of free inodes
300 arg_info = required_arg
305 The minimum amount of free inodes on the file system containing the
306 destination dir. If the percentage of free inodes drops below the
307 given value, snapshot removal kicks in like in case of low disk space.
309 The number of free inodes is determined from the f_ffree field of
310 the statvfs structure. However, some file systems set this field to
311 zero, indicating that the number of inodes is basically unlimited.
312 Moreover it is not possible to reliably detect whether this is the
313 case. Therefore this feature is disabled by default. It's safe to
314 enable it for ext2/ext3/ext4 file systems on linux though.
316 A value of zero (the default) deactivates this check.
318 [option keep-redundant]
320 summary = prune by disk space only
322 By default, redundant and outdated snapshots are removed automatically
323 to keep the number of snapshots in harmony with the configured
324 policy. If this flag is given, dss removes such snapshots only if
325 disk space or number of free inodes becomes low.
327 [option min-complete]
328 summary = minimal number of complete snapshots to keep
329 arg_info = optional_arg
334 This option is only relevant if snapshots must be deleted because
337 dss refuses to remove old snapshots if there are fewer complete
338 snapshots left than the given number. The default value of one
339 guarantees that at least one complete snapshot is available at
342 If only <num> complete snapshots are left, and there is not enough
343 disk space available for another snapshot, the program terminates
344 with a "No space left on device" error.
348 dss supports the subcommands described below. If no subcommand is
349 given, the list of available subcommands is shown and the program
350 terminates successfully without performing any further action.
354 purpose = start creating and pruning snapshots
356 This is the main mode of operation. Snapshots are created in an endless
357 loop as needed and pruned automatically. The loop only terminates on
358 fatal errors or if a terminating signal was received. See also the
363 summary = run as background daemon
365 If this option is given, the dss command detaches from the console
366 and continues to run in the background. It is not possible to let
367 a daemonized process re-attach to the console by editing the config
368 file and sending SIGHUP. However, log output may be redirected to a
369 different file in this way.
375 summary = where to write log output
376 arg_info = required_arg
379 default_val = /dev/null
381 This option is only honored if --daemon is given, in which case
382 log messages go to the given file. Otherwise the option is silently
383 ignored and log output is written to stderr.
385 [option max-rsync-errors]
386 summary = terminate after this many rsync failures
388 arg_info = required_arg
392 If the rsync process exits with a fatal error, dss restarts the command
393 in the hope that the problem is transient and subsequent rsync runs
394 succeed. After the given number of consecutive rsync error exits,
395 however, dss gives up, executes the exit hook and terminates. Set
396 this to zero if dss should exit immediately on the first rsync error.
398 The only non-fatal error is when rsync exits with code 24. This
399 indicates a partial transfer due to vanished source files and happens
400 frequently when snapshotting a directory which is concurrently being
404 purpose = execute rsync once to create a new snapshot
406 This command does not check the amount free disk space. The pre-create
407 and post-create hooks are honored, however.
409 Specify --dry-run to see the rsync command which is executed to create
413 purpose = remove redundant and outdated snapshots
415 A snapshot is considered outdated if its interval number is greater or
416 equal than the specified number of unit intervals. See --unit-interval
417 and --num-intervals above.
419 A snapshot is said to be redundant if the interval it belongs to
420 contains more than the configured number of snapshots.
422 The prune command gets rid of both outdated and redundant snapshots. At
423 most one snapshot is removed per invocation. If --dry-run is given, the
424 subcommand only prints the snapshot that would be removed.
427 purpose = print the list of all snapshots
429 The list contains all existing snapshots, no matter of their state.
430 Incomplete snapshots and snapshots being deleted will also be listed.
433 purpose = send a signal to a running dss process
435 This sends a signal to the dss process that corresponds to the given
436 config file. If --dry-run is given, the PID of the dss process is
437 written to stdout, but no signal is sent.
441 summary = send the given signal rather than SIGTERM
443 arg_info = required_arg
445 default_val = SIGTERM
447 Like for kill(1), alternate signals may be specified in three ways: as
448 a signal number (e.g., 9), the signal name (e.g., KILL), or the signal
449 name prefixed with "SIG" (e.g., SIGKILL). In the latter two forms,
450 the signal name and the prefix are case insensitive, so "sigkill"
453 Sending SIGHUP causes the running dss process to reload its config file.
457 Written by Andre Noll
459 Copyright (C) 2008 - present Andre Noll
461 License: GNU GPL version 2
463 This is free software: you are free to change and redistribute it.
465 There is NO WARRANTY, to the extent permitted by law.
468 .MT <maan@tuebingen.mpg.de>