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.
83 summary = abort if destination directory is not a mountpoint
85 This option checks whether a file system is mounted on the directory
86 specified as the argument to --dest-dir. Operation proceeds only
87 if this is the case. Otherwise dss exits unsuccessfully without
88 performing any action. Use this option to prevent snapshot creation
89 if the snapshot file system is not mounted.
91 This option is silently ignored for subcommands which do not depend
92 on the destination directory.
94 [option Rsync-options]
95 summary = Controlling how rsync is run
98 These options are only relevant to the run and the create subcommands.
102 summary = host to take snapshots from
104 arg_info = required_arg
106 default_val = localhost
108 If this option is given and its value differs from the local
109 host, then rsync uses ssh. Make sure there is no password
110 needed for the ssh connection. To achieve that, use public key
111 authentication for ssh and, if needed, set the remote user name
112 by using the --remote-user option.
116 summary = Remote user name (default: current user)
117 arg_info = required_arg
121 Set this if the user that runs dss is different from the user on the
125 summary = run rsync with --checksum occasionally
127 arg_info = required_arg
131 If a file on the backup becomes corrupt in a way that file size
132 and modification time still match the original file, rsync will not
133 consider the file for transfer ("quick check"). Hence the corruption
134 stays on the backup until the file is modified on the source.
135 The --checksum option of rsync disables the quick check and compares
136 the contents of each file, fixing such corruptions. Since computing
137 the checksums adds a significant slowdown due to a lot of disk I/O,
138 the option is not enabled by default.
140 The argument to the --checksum option of dss is a number between 0
141 and 1000, inclusively, which determines the probability of adding
142 --checksum to the rsync options each time a snapshot is created. The
143 default value zero means to never add the option. The value 100 will
144 create every tenth snapshot (on average) using checksums, and the
145 value 1000 will always pass --checksum to rsync.
147 [option rsync-option]
149 summary = further rsync options
151 arg_info = required_arg
155 This option may be given multiple times. The given argument is
156 passed verbatim to the rsync command. Note that in order to use
157 rsync options that require an argument, you have to specify the
158 option and its argument as separate --rsync-options, like this:
160 --rsync-option --exclude --rsync-option /proc
163 summary = Fine tuning the number of snapshots per time unit
166 Snapshot aging is implemented in terms of intervals. There are two
167 command line options related to intervals: the duration u of a unit
168 interval and the number of unit intervals, denoted n below.
170 dss removes snapshots older than n times u and tries to keep 2^(n -
171 k - 1) snapshots in interval k, where the interval number k counts
172 from zero to n - 1, with zero being the most recent unit interval.
174 Hence the oldest snapshot will at most be u * n days old (4 days *
175 5 intervals = 20 days, if default values are used). Moreover, there
176 are at most 2^n - 1 snapshots in total (2^5 - 1 = 31 by default). Note
177 that for this to work out your system must be fast enough to create at
178 least 2^(n - 1) snapshots per unit interval (16 snapshots in 4 days =
179 one snapshot in 6 hours), because this is the number of snapshots in
182 [option unit-interval]
184 summary = the duration of a unit interval
186 arg_info = required_arg
190 Increasing this number instructs dss to create fewer snapshots per
191 time unit while the number of snapshots to keep stays the same.
193 [option num-intervals]
195 summary = the number of unit intervals
197 arg_info = required_arg
201 Increasing this number by one doubles the total number of
205 summary = Commands to be run on certain events
208 All hooks default to "true". That is, the true(1) utility (which
209 always returns with exit code zero) is executed if the hook command
212 [option pre-create-hook]
214 summary = executed before a snapshot is created
216 arg_info = required_arg
220 This command is executed before dss runs rsync to create a new
221 snapshot. If the command returns with a non-zero exit status, no
222 snapshot will be created and the operation is retried later.
224 For example, the command could execute a script that checks whether
225 all snapshot-related file systems are mounted.
227 Another possible application of the pre-create hook is to return
228 non-zero during office hours in order to not slow down the file
229 systems by taking snapshots.
231 [option post-create-hook]
232 summary = executed after a snapshot has been created
234 arg_info = required_arg
238 This is only executed if a snapshot has successfully been created. The
239 full path of the newly created snapshot is passed to the hook as the
240 first argument. The exit code of this hook is ignored.
242 For instance this hook could count the number of files per user
243 and/or compute disk usage patterns to be stored in a database for
246 [option pre-remove-hook]
247 summary = executed before a snapshot is removed
249 arg_info = required_arg
253 The full path to the snapshot which is about to be removed is passed
254 to the command as the first argument. If the command returns with
255 a non-zero exit status, the snapshot is not going to be removed and
256 the operation is retried later.
258 For example, one could execute a script that checks whether the
259 snapshot to be deleted is currently used by another process, e.g. by
260 a tape-based backup system that runs concurrently to dss.
262 Another possible application of this is to record disk-usage
263 patterns before and after snapshot removal.
265 [option post-remove-hook]
266 summary = executed after snapshot removal
268 arg_info = required_arg
272 As for the pre-remove hook, the full path of the removed snapshot is
273 passed to the hook as the first argument. The exit code of this hook
277 summary = executed before the run command exits
279 arg_info = required_arg
283 This hook is only relevant to the run subcommand. It is executed just
284 before dss terminates. The reason for termination is passed as the
287 One possible application for this hook is to send email to the system
288 administrator to let her know that no more snapshots are going to
292 [option disk-space-monitoring]
293 summary = Disk space monitoring
296 The options of this section control the aggressiveness of snapshot
297 removal. That is, they define under which circumstances existing
298 snapshots are removed. These options are only relevant to the run
299 and the prune subcommands.
303 summary = minimal amount of free disk space
304 arg_info = required_arg
309 If disk space on the file system containing the destination
310 directory gets low, the run subcommand suspends the currently
311 running rsync process and starts to remove snapshots in order to
312 free disk space. This option specifies the minimal amount of free
313 disk space. If less than the given number of megabytes is available,
314 snapshots are being deleted. See also the --min_free_percent and the
315 min-free-percent-inodes options below.
317 A value of zero deactivates this check.
319 [option min-free-percent]
321 summary = minimal percentage of free disk space
322 arg_info = required_arg
327 This is like --min-free-mb but allows to specify the amount of
328 free disk space as a percentage. It is not recommended to set both
329 --min-free-mb and --min-free-percent to zero as this will cause your
330 file system to fill up quickly.
332 [option min-free-percent-inodes]
334 summary = minimal percent of free inodes
335 arg_info = required_arg
340 The minimum amount of free inodes on the file system containing the
341 destination dir. If the percentage of free inodes drops below the
342 given value, snapshot removal kicks in like in case of low disk space.
344 The number of free inodes is determined from the f_ffree field of
345 the statvfs structure. However, some file systems set this field to
346 zero, indicating that the number of inodes is basically unlimited.
347 Moreover it is not possible to reliably detect whether this is the
348 case. Therefore this feature is disabled by default. It's safe to
349 enable it for ext2/ext3/ext4 file systems on linux though.
351 A value of zero (the default) deactivates this check.
353 [option keep-redundant]
355 summary = prune by disk space only
357 By default, redundant and outdated snapshots are removed automatically
358 to keep the number of snapshots in harmony with the configured
359 policy. If this flag is given, dss removes such snapshots only if
360 disk space or number of free inodes becomes low.
362 [option min-complete]
363 summary = minimal number of complete snapshots to keep
364 arg_info = optional_arg
369 This option is only relevant if snapshots must be deleted because
372 dss refuses to remove old snapshots if there are fewer complete
373 snapshots left than the given number. The default value of one
374 guarantees that at least one complete snapshot is available at
377 If only <num> complete snapshots are left, and there is not enough
378 disk space available for another snapshot, the program terminates
379 with a "No space left on device" error.
383 dss supports the subcommands described below. If no subcommand is
384 given, the list of available subcommands is shown and the program
385 terminates successfully without performing any further action.
389 purpose = start creating and pruning snapshots
391 This is the main mode of operation. Snapshots are created in an endless
392 loop as needed and pruned automatically. The loop only terminates on
393 fatal errors or if a terminating signal was received. See also the
398 summary = run as background daemon
400 If this option is given, the dss command detaches from the console
401 and continues to run in the background. It is not possible to let
402 a daemonized process re-attach to the console by editing the config
403 file and sending SIGHUP. However, log output may be redirected to a
404 different file in this way.
410 summary = where to write log output
411 arg_info = required_arg
414 default_val = /dev/null
416 This option is only honored if --daemon is given, in which case
417 log messages go to the given file. Otherwise the option is silently
418 ignored and log output is written to stderr.
420 [option max-rsync-errors]
421 summary = terminate after this many rsync failures
423 arg_info = required_arg
427 If the rsync process exits with a fatal error, dss restarts the command
428 in the hope that the problem is transient and subsequent rsync runs
429 succeed. After the given number of consecutive rsync error exits,
430 however, dss gives up, executes the exit hook and terminates. Set
431 this to zero if dss should exit immediately on the first rsync error.
433 The only non-fatal error is when rsync exits with code 24. This
434 indicates a partial transfer due to vanished source files and happens
435 frequently when snapshotting a directory which is concurrently being
439 purpose = execute rsync once to create a new snapshot
441 This command does not check the amount free disk space. The pre-create
442 and post-create hooks are honored, however.
444 Specify --dry-run to see the rsync command which is executed to create
448 purpose = remove redundant and outdated snapshots
450 A snapshot is considered outdated if its interval number is greater or
451 equal than the specified number of unit intervals. See --unit-interval
452 and --num-intervals above.
454 A snapshot is said to be redundant if the interval it belongs to
455 contains more than the configured number of snapshots.
457 The prune command gets rid of both outdated and redundant snapshots. At
458 most one snapshot is removed per invocation. If --dry-run is given, the
459 subcommand only prints the snapshot that would be removed.
462 purpose = print the list of all snapshots
464 The list contains all existing snapshots, no matter of their state.
465 Incomplete snapshots and snapshots being deleted will also be listed.
468 purpose = send a signal to a running dss process
470 This sends a signal to the dss process that corresponds to the given
471 config file. If --dry-run is given, the PID of the dss process is
472 written to stdout, but no signal is sent.
476 summary = send the given signal rather than SIGTERM
478 arg_info = required_arg
480 default_val = SIGTERM
482 Like for kill(1), alternate signals may be specified in three ways: as
483 a signal number (e.g., 9), the signal name (e.g., KILL), or the signal
484 name prefixed with "SIG" (e.g., SIGKILL). In the latter two forms,
485 the signal name and the prefix are case insensitive, so "sigkill"
488 Sending SIGHUP causes the running dss process to reload its config file.
492 Written by Andre Noll
494 Copyright (C) 2008 - present Andre Noll
496 License: GNU GPL version 2
498 This is free software: you are free to change and redistribute it.
500 There is NO WARRANTY, to the extent permitted by law.
503 .MT <maan@tuebingen.mpg.de>