Removing backup snapshots

All backup space is finite, so restic allows removing old snapshots. This can be done either manually (by specifying a snapshot ID to remove) or by using a policy that describes which snapshots to forget. For all remove operations, two commands need to be called in sequence: forget to remove snapshots, and prune to remove the remaining data that was referenced only by the removed snapshots. The latter can be automated with the --prune option of forget, which runs prune automatically if any snapshots were actually removed.

Pruning snapshots can be a time-consuming process, depending on the number of snapshots and data to process. During a prune operation, the repository is locked and backups cannot be completed. Please plan your pruning so that there’s time to complete it and it doesn’t interfere with regular backup runs.

It is advisable to run restic check after pruning, to make sure you are alerted, should the internal data structures of the repository be damaged.

Remove a single snapshot

The command snapshots can be used to list all snapshots in a repository like this:

$ restic -r /srv/restic-repo snapshots
enter password for repository:
ID        Date                 Host      Tags  Directory
----------------------------------------------------------------------
40dc1520  2015-05-08 21:38:30  kasimir         /home/user/work
79766175  2015-05-08 21:40:19  kasimir         /home/user/work
bdbd3439  2015-05-08 21:45:17  luigi           /home/art
590c8fc8  2015-05-08 21:47:38  kazik           /srv
9f0bc19e  2015-05-08 21:46:11  luigi           /srv

In order to remove the snapshot of /home/art, use the forget command and specify the snapshot ID on the command line:

$ restic -r /srv/restic-repo forget bdbd3439
enter password for repository:
removed snapshot bdbd3439

Afterwards this snapshot is removed:

$ restic -r /srv/restic-repo snapshots
enter password for repository:
ID        Date                 Host     Tags  Directory
----------------------------------------------------------------------
40dc1520  2015-05-08 21:38:30  kasimir        /home/user/work
79766175  2015-05-08 21:40:19  kasimir        /home/user/work
590c8fc8  2015-05-08 21:47:38  kazik          /srv
9f0bc19e  2015-05-08 21:46:11  luigi          /srv

But the data that was referenced by files in this snapshot is still stored in the repository. To cleanup unreferenced data, the prune command must be run:

$ restic -r /srv/restic-repo prune
enter password for repository:
repository 33002c5e opened successfully, password is correct
loading all snapshots...
loading indexes...
finding data that is still in use for 4 snapshots
[0:00] 100.00%  4 / 4 snapshots
searching used packs...
collecting packs for deletion and repacking
[0:00] 100.00%  5 / 5 packs processed

to repack:            69 blobs / 1.078 MiB
this removes:         67 blobs / 1.047 MiB
to delete:             7 blobs / 25.726 KiB
total prune:          74 blobs / 1.072 MiB
remaining:            16 blobs / 38.003 KiB
unused size after prune: 0 B (0.00% of remaining size)

repacking packs
[0:00] 100.00%  2 / 2 packs repacked
rebuilding index
[0:00] 100.00%  3 / 3 packs processed
deleting obsolete index files
[0:00] 100.00%  3 / 3 files deleted
removing 3 old packs
[0:00] 100.00%  3 / 3 files deleted
done

Afterwards the repository is smaller.

You can automate this two-step process by using the --prune switch to forget:

$ restic forget --keep-last 1 --prune
snapshots for host mopped, directories /home/user/work:

keep 1 snapshots:
ID        Date                 Host        Tags        Directory
----------------------------------------------------------------------
4bba301e  2017-02-21 10:49:18  mopped                  /home/user/work

remove 1 snapshots:
ID        Date                 Host        Tags        Directory
----------------------------------------------------------------------
8c02b94b  2017-02-21 10:48:33  mopped                  /home/user/work

1 snapshots have been removed, running prune
loading all snapshots...
loading indexes...
finding data that is still in use for 1 snapshots
[0:00] 100.00%  1 / 1 snapshots
searching used packs...
collecting packs for deletion and repacking
[0:00] 100.00%  5 / 5 packs processed

to repack:           69 blobs / 1.078 MiB
this removes         67 blobs / 1.047 MiB
to delete:            7 blobs / 25.726 KiB
total prune:         74 blobs / 1.072 MiB
remaining:           16 blobs / 38.003 KiB
unused size after prune: 0 B (0.00% of remaining size)

repacking packs
[0:00] 100.00%  2 / 2 packs repacked
rebuilding index
[0:00] 100.00%  3 / 3 packs processed
deleting obsolete index files
[0:00] 100.00%  3 / 3 files deleted
removing 3 old packs
[0:00] 100.00%  3 / 3 files deleted
done

Removing snapshots according to a policy

Removing snapshots manually is tedious and error-prone, therefore restic allows specifying a policy (one or more --keep-* options) for which snapshots to keep. You can for example define how many hourly, daily, weekly, monthly and yearly snapshots to keep, and any other snapshots will be removed.

Warning

If you use an append-only repository with policy-based snapshot removal, some security considerations are important. Please refer to the section below for more information.

Note

You can always use the --dry-run option of the forget command, which instructs restic to not remove anything but instead just print what actions would be performed.

The forget command accepts the following policy options:

  • --keep-last n keep the n last (most recent) snapshots.
  • --keep-hourly n for the last n hours which have one or more snapshots, keep only the most recent one for each hour.
  • --keep-daily n for the last n days which have one or more snapshots, keep only the most recent one for each day.
  • --keep-weekly n for the last n weeks which have one or more snapshots, keep only the most recent one for each week.
  • --keep-monthly n for the last n months which have one or more snapshots, keep only the most recent one for each month.
  • --keep-yearly n for the last n years which have one or more snapshots, keep only the most recent one for each year.
  • --keep-tag keep all snapshots which have all tags specified by this option (can be specified multiple times).
  • --keep-within duration keep all snapshots having a timestamp within the specified duration of the latest snapshot, where duration is a number of years, months, days, and hours. E.g. 2y5m7d3h will keep all snapshots made in the two years, five months, seven days and three hours before the latest (most recent) snapshot.
  • --keep-within-hourly duration keep all hourly snapshots made within the specified duration of the latest snapshot. The duration is specified in the same way as for --keep-within and the method for determining hourly snapshots is the same as for --keep-hourly.
  • --keep-within-daily duration keep all daily snapshots made within the specified duration of the latest snapshot.
  • --keep-within-weekly duration keep all weekly snapshots made within the specified duration of the latest snapshot.
  • --keep-within-monthly duration keep all monthly snapshots made within the specified duration of the latest snapshot.
  • --keep-within-yearly duration keep all yearly snapshots made within the specified duration of the latest snapshot.

Note

All calendar related options (--keep-{hourly,daily,...}) work on natural time boundaries and not relative to when you run forget. Weeks are Monday 00:00 to Sunday 23:59, days 00:00 to 23:59, hours :00 to :59, etc. They also only count hours/days/weeks/etc which have one or more snapshots.

Note

All duration related options (--keep-{within,-*}) ignore snapshots with a timestamp in the future (relative to when the forget command is run) and these snapshots will hence not be removed.

Note

Specifying --keep-tag '' will match untagged snapshots only.

When forget is run with a policy, restic first loads the list of all snapshots and groups them by their host name and paths. The grouping options can be set with --group-by, e.g. using --group-by paths,tags to instead group snapshots by paths and tags. The policy is then applied to each group of snapshots individually. This is a safety feature to prevent accidental removal of unrelated backup sets. To disable grouping and apply the policy to all snapshots regardless of their host, paths and tags, use --group-by '' (that is, an empty value to --group-by).

Additionally, you can restrict the policy to only process snapshots which have a particular hostname with the --host parameter, or tags with the --tag option. When multiple tags are specified, only the snapshots which have all the tags are considered. For example, the following command removes all but the latest snapshot of all snapshots that have the tag foo:

$ restic forget --tag foo --keep-last 1

This command removes all but the last snapshot of all snapshots that have either the foo or bar tag set:

$ restic forget --tag foo --tag bar --keep-last 1

To only keep the last snapshot of all snapshots with both the tag foo and bar set use:

$ restic forget --tag foo,bar --keep-last 1

To ensure only untagged snapshots are considered, specify the empty string ‘’ as the tag.

$ restic forget --tag '' --keep-last 1

Let’s look at a simple example: Suppose you have only made one backup every Sunday for 12 weeks:

$ restic snapshots
repository f00c6e2a opened successfully, password is correct
ID        Time                 Host        Tags        Paths
---------------------------------------------------------------
0a1f9759  2019-09-01 11:00:00  mopped                  /home/user/work
46cfe4d5  2019-09-08 11:00:00  mopped                  /home/user/work
f6b1f037  2019-09-15 11:00:00  mopped                  /home/user/work
eb430a5d  2019-09-22 11:00:00  mopped                  /home/user/work
8cf1cb9a  2019-09-29 11:00:00  mopped                  /home/user/work
5d33b116  2019-10-06 11:00:00  mopped                  /home/user/work
b9553125  2019-10-13 11:00:00  mopped                  /home/user/work
e1a7b58b  2019-10-20 11:00:00  mopped                  /home/user/work
8f8018c0  2019-10-27 11:00:00  mopped                  /home/user/work
59403279  2019-11-03 11:00:00  mopped                  /home/user/work
dfee9fb4  2019-11-10 11:00:00  mopped                  /home/user/work
e1ae2f40  2019-11-17 11:00:00  mopped                  /home/user/work
---------------------------------------------------------------
12 snapshots

Then forget --keep-daily 4 will keep the last four snapshots, for the last four Sundays, and remove the other snapshots:

$ restic forget --keep-daily 4 --dry-run
repository f00c6e2a opened successfully, password is correct
Applying Policy: keep the last 4 daily snapshots
keep 4 snapshots:
ID        Time                 Host        Tags        Reasons         Paths
-------------------------------------------------------------------------------
8f8018c0  2019-10-27 11:00:00  mopped                  daily snapshot  /home/user/work
59403279  2019-11-03 11:00:00  mopped                  daily snapshot  /home/user/work
dfee9fb4  2019-11-10 11:00:00  mopped                  daily snapshot  /home/user/work
e1ae2f40  2019-11-17 11:00:00  mopped                  daily snapshot  /home/user/work
-------------------------------------------------------------------------------
4 snapshots

remove 8 snapshots:
ID        Time                 Host        Tags        Paths
---------------------------------------------------------------
0a1f9759  2019-09-01 11:00:00  mopped                  /home/user/work
46cfe4d5  2019-09-08 11:00:00  mopped                  /home/user/work
f6b1f037  2019-09-15 11:00:00  mopped                  /home/user/work
eb430a5d  2019-09-22 11:00:00  mopped                  /home/user/work
8cf1cb9a  2019-09-29 11:00:00  mopped                  /home/user/work
5d33b116  2019-10-06 11:00:00  mopped                  /home/user/work
b9553125  2019-10-13 11:00:00  mopped                  /home/user/work
e1a7b58b  2019-10-20 11:00:00  mopped                  /home/user/work
---------------------------------------------------------------
8 snapshots

The processed snapshots are evaluated against all --keep-* options but a snapshot only need to match a single option to be kept (the results are ORed). This means that the most recent snapshot on a Sunday would match both hourly, daily and weekly --keep-* options, and possibly more depending on calendar.

For example, suppose you make one backup every day for 100 years. Then forget --keep-daily 7 --keep-weekly 5 --keep-monthly 12 --keep-yearly 75 would keep the most recent 7 daily snapshots and 4 last-day-of-the-week ones (since the 7 dailies already include 1 weekly). Additionally, 12 or 11 last-day-of-the-month snapshots will be kept (depending on whether one of them ends up being the same as a daily or weekly). And finally 75 or 74 last-day-of-the-year snapshots are kept, depending on whether one of them ends up being the same as an already kept snapshot. All other snapshots are removed.

You might want to maintain the same policy as in the example above, but have irregular backups. For example, the 7 snapshots specified with --keep-daily 7 might be spread over a longer period. If what you want is to keep daily snapshots for the last week, weekly for the last month, monthly for the last year and yearly for the last 75 years, you can instead specify forget --keep-within-daily 7d --keep-within-weekly 1m --keep-within-monthly 1y --keep-within-yearly 75y (note that 1w is not a recognized duration, so you will have to specify 7d instead).

For safety reasons, restic refuses to act on an “empty” policy. For example, if one were to specify --keep-last 0 to forget all snapshots in the repository, restic will respond that no snapshots will be removed. To delete all snapshots, use --keep-last 1 and then finally remove the last snapshot manually (by passing the ID to forget).

Security considerations in append-only mode

Note

TL;DR: With append-only repositories, one should specifically use the --keep-within option of the forget command when removing snapshots.

To prevent a compromised backup client from deleting its backups (for example due to a ransomware infection), a repository service/backend can serve the repository in a so-called append-only mode. This means that the repository is served in such a way that it can only be written to and read from, while delete and overwrite operations are denied. Restic’s rest-server features an append-only mode, but few other standard backends do. To support append-only with such backends, one can use rclone as a complement in between the backup client and the backend service.

To remove snapshots and recover the corresponding disk space, the forget and prune commands require full read, write and delete access to the repository. If an attacker has this, the protection offered by append-only mode is naturally void. The usual and recommended setup with append-only repositories is therefore to use a separate and well-secured client whenever full access to the repository is needed, e.g. for administrative tasks such as running forget, prune and other maintenance commands.

However, even with append-only mode active and a separate, well-secured client used for administrative tasks, an attacker who is able to add garbage snapshots to the repository could bring the snapshot list into a state where all the legitimate snapshots risk being deleted by an unsuspecting administrator that runs the forget command with certain --keep-* options, leaving only the attacker’s useless snapshots.

For example, if the forget policy is to keep three weekly snapshots, and the attacker adds an empty snapshot for each of the last three weeks, all with a timestamp (see the backup command’s --time option) slightly more recent than the existing snapshots (but still within the target week), then the next time the repository administrator (or a scheduled job) runs the forget command with this policy, the legitimate snapshots will be removed (since the policy will keep only the most recent snapshot within each week). Even without running prune, recovering data would be messy and some metadata lost.

To avoid this, forget policies applied to append-only repositories should use the --keep-within option, as this will keep not only the attacker’s snapshots but also the legitimate ones. Assuming the system time is correctly set when forget runs, this will allow the administrator to notice problems with the backup or the compromised host (e.g. by seeing more snapshots than usual or snapshots with suspicious timestamps). This is, of course, limited to the specified duration: if forget --keep-within 7d is run 8 days after the last good snapshot, then the attacker can still use that opportunity to remove all legitimate snapshots.

Customize pruning

To understand the custom options, we first explain how the pruning process works:

  1. All snapshots and directories within snapshots are scanned to determine which data is still in use.

  2. For all files in the repository, restic finds out if the file is fully used, partly used or completely unused.

  3. Completely unused files are marked for deletion. Fully used files are kept. A partially used file is either kept or marked for repacking depending on user options.

    Note that for repacking, restic must download the file from the repository storage and re-upload the needed data in the repository. This can be very time-consuming for remote repositories.

  4. After deciding what to do, prune will actually perform the repack, modify the index according to the changes and delete the obsolete files.

The prune command accepts the following options:

  • --max-unused limit allow unused data up to the specified limit within the repository. This allows restic to keep partly used files instead of repacking them.

    The limit can be specified in several ways:

    • As an absolute size (e.g. 200M). If you want to minimize the space used by your repository, pass 0 to this option.
    • As a size relative to the total repo size (e.g. 10%). This means that after prune, at most 10% of the total data stored in the repo may be unused data. If the repo after prune has a size of 500MB, then at most 50MB may be unused.
    • If the string unlimited is passed, there is no limit for partly unused files. This means that as long as some data is still used within a file stored in the repo, restic will just leave it there. Use this if you want to minimize the time and bandwidth used by the prune operation. Note that metadata will still be repacked.

    Restic tries to repack as little data as possible while still ensuring this limit for unused data. The default value is 5%.

  • --max-repack-size size if set limits the total size of files to repack. As prune first stores all repacked files and deletes the obsolete files at the end, this option might be handy if you expect many files to be repacked and fear to run low on storage.

  • --repack-cacheable-only if set to true only files which contain metadata and would be stored in the cache are repacked. Other pack files are not repacked if this option is set. This allows a very fast repacking using only cached data. It can, however, imply that the unused data in your repository exceeds the value given by --max-unused. The default value is false.

  • --dry-run only show what prune would do.

  • --verbose increased verbosity shows additional statistics for prune.

Recovering from “no free space” errors

In some cases when a repository has grown large enough to fill up all disk space or the allocated quota, then prune might fail to free space. prune works in such a way that a repository remains usable no matter at which point the command is interrupted. However, this also means that prune requires some scratch space to work.

In most cases it is sufficient to instruct prune to use as little scratch space as possible by running it as prune --max-repack-size 0. Note that for restic versions before 0.13.0 prune --max-repack-size 1 must be used. Obviously, this can only work if several snapshots have been removed using forget before. This then allows the prune command to actually remove data from the repository. If the command succeeds, but there is still little free space, then remove a few more snapshots and run prune again.

If prune fails to complete, then prune --unsafe-recover-no-free-space SOME-ID is available as a method of last resort. It allows prune to work with little to no free space. However, a failed prune run can cause the repository to become temporarily unusable. Therefore, make sure that you have a stable connection to the repository storage, before running this command. In case the command fails, it may become necessary to manually remove all files from the index/ folder of the repository and run rebuild-index afterwards.

To prevent accidental usages of the --unsafe-recover-no-free-space option it is necessary to first run prune --unsafe-recover-no-free-space SOME-ID and then replace SOME-ID with the requested ID.