borg prune visualized (count and interval mixed)
================================================================================

Scenario: You use borg to perform daily backups. As backups age, the day-to-day
changes become less important, so to save storage space you want older archives
to "thin out" over time while retaining most recent archives. Your backup
script runs `borg create`, immediately followed by `borg prune`.

Assume today is 2026-06-04 and you always start your backups at 16:00. You have
been creating backup archives starting at 16:00, followed by pruning, on most
days going back to late 2025. Today, `borg create` took a little longer than
usual. It's 16:12 now and you run `borg prune`.

You want Borg to keep one archive per day for one week, four weekly archives,
one archive per month for five months, and two yearly backups. For that, you
use the following command:

```
borg prune \
  --keep-daily 1w \
  --keep-weekly 4 \
  --keep-monthly 5m \
  --keep-yearly 2 \
  --from '2026-06-04 16:00'
```

The `--keep-*` options reflect the intended retention policy exactly. Note the
different wording in the retention policy for weekly and yearly archives: They
aren't *interval*-based, but *count*-based.

Another important detail here is `--from`. Without it, intervals would be
calculated relative to the actual start time of `prune` - in this case 16:12.
Since your backups are always created at 16:00, this 12-minute shift would move
the cutoff point of intervals and could cause archives near the boundary to
unexpectedly fall outside the expected time window.

By specifying `--from '2026-06-04 16:00'`, all intervals are anchored to the
intended reference time (16:00), not the moment `prune` happens to run. This
ensures stable and predictable retention behavior, independent of when `prune`
actually runs.

The first archive was made on 2025-11-15. You missed the backups on 2026-03-31,
2026-05-24, and 2026-06-03.

Below you find an overview of what archives `prune` will keep.

Archives kept by the `--keep-daily` rule are marked by a "d" to the right,
archives kept by the `--keep-weekly` rule are marked by a "w" to the right,
archives kept by the `--keep-monthly` rule are marked by a "m" to the right,
archives kept by the `--keep-yearly` rule are marked by a "y" to the right, and
archives kept by the `--from` rule are marked by a "x" to the right.


Calendar view
-------------
                            2025
                            November              December
                                             1  2  3  4  5  6  7
                                             8  9 10 11 12 13 14
                                     15y16  15 16 17 18 19 20 21
                      17 18 19 20 21 22 23  22 23 24 25 26 27 28
                      24 25 26 27 28 29 30  29 30 31m

                            2026
      January               February               March
          1  2  3  4                     1                     1
 5  6  7  8  9 10 11   2  3  4  5  6  7  8   2  3  4  5  6  7  8
12 13 14 15 16 17 18   9 10 11 12 13 14 15   9 10 11 12 13 14 15
19 20 21 22 23 24 25  16 17 18 19 20 21 22  16 17 18 19 20 21 22
26 27 28 29 30 31m    23 24 25 26 27 28m    23 24 25 26 27 28 29
                                            30m31

       April                  May                   June
       1  2  3  4  5               1  2  3w  1d 2d 3  4x
 6  7  8  9 10 11 12   4  5  6  7  8  9 10w
13 14 15 16 17 18 19  11 12 13 14 15 16 17w
20 21 22 23 24 25 26  18 19 20 21 22 23w24
27 28 29 30m          25 26 27 28d29d30d31d


List view
---------

--keep-daily 1w     --keep-weekly 4     --keep-monthly 5m   --keep-yearly 2
--------------------------------------------------------------------------------
                                                            1. 2025-11-15 (oldest)
1. 2026-06-02       1. 2026-05-23       1. 2026-04-30
2. 2026-06-01       2. 2026-05-17       2. 2026-03-30
3. 2026-05-31       3. 2026-05-10       3. 2026-02-28
4. 2026-05-30       4. 2026-05-03       4. 2026-01-31
5. 2026-05-29                           5. 2025-12-31
6. 2026-05-28

2026-06-04 is additionally kept due to `--from`.


Notes
-----

The current day's archive is always kept, because `create` ran after the date
given with `--from`. For `prune`, it's as if this archive doesn't exist (yet).

2026-06-03 was skipped, so no archive can be kept with `--keep-daily` for that
day. Other than with a *count*-based policy, no compensation is made for an
*interval* like `--keep-daily 1w`, so the rule simply keeps one archive fewer.

2026-05-28 16:00 is exactly one week before `--from`. Since `create` always
runs after 16:00, the archive created on 2026-05-28 is kept, too. Without
`--from`, Borg would cut off at 2026-05-28 16:12 instead, which would likely
mean that the archive created on 2026-05-28 would be pruned. `--from` ensures
that 2026-05-28 is consistently kept. If you want it consistently pruned, try a
later reference time, e.g. `--from '2026-06-04 23:59:59'`.

2026-05-31 is considered not only by `--keep-daily`, but by `--keep-weekly`
and `--keep-monthly`, too. The archive is effectively kept by `--keep-daily`,
but how this affects other rules differs between *count*- and *interval*-based
policies. For *interval*-based rules like `--keep-monthly 5m` it has no effect:
The rule simply keeps one archive fewer in that case.

For *count*-based rules like `--keep-weekly 4` it has an effect: The policy
tells Borg to keep 4 weekly archives. Weekly slots are identified by ISO week
number; in this example the four kept archives cover weeks 18 through 21. If
2026-05-31 is kept by another rule already, Borg compensates by keeping an
older archive instead. Consequently, Borg will also keep the 2026-05-03 archive.

The week 22 slot is consumed by that same 2026-05-31 archive. Any other
archive from that week is not kept by weekly, even if it falls within the
daily window boundary. This is why 2026-05-27 is pruned: it has just fallen
out of the daily window, and no other rule covers it.

Since 2026-05-24 and 2026-03-31 were skipped, Borg substitutes the next best
candidate for each: 2026-05-23 fills the week 21 weekly slot, and 2026-03-30
fills the March monthly slot.

The implementation of `--keep-monthly 5m` is somewhat special: Borg defines a
month as a fixed 31-day period, independent of the actual calendar dates
involved. As a result, `5m` corresponds to 5 × 31 = 155 days. The archive from
2025-12-31 16:00 is exactly 155 days older than the reference time and is
therefore retained by `--keep-monthly`.

As a result, there are no true yearly candidates. In the absence of a better
candidate, `--keep-yearly 2` only matches the oldest archive, 2025-11-15.

Since interval rules define time windows rather than competing for a fixed
number of slots, their interplay is simpler than count-based rules. An archive
is kept by an interval rule as long as it falls within the specified window;
the next rule simply considers whatever remains.

Intervals and counts can be mixed freely. Yearly retention in this example is
done by retention count instead of intervals. A count rule paired with interval
rules behaves just as it would if all preceding rules were also counts:
Archives already kept by earlier rules are excluded from consideration. In this
example, every yearly slot is consumed by earlier rules; no true yearly
candidate remains, so the oldest archive at 2025-11-15 is kept to preserve
rolling backup semantics.
