Pruning Policies

In zrepl, pruning means destroying snapshots. Pruning must happen on both sides of a replication or the systems would inevitable run out of disk space at some point.

Typically, the requirements to temporal resolution and maximum retention time differ per side. For example, when using zrepl to back up a busy database server, you will want high temporal resolution (snapshots every 10 min) for the last 24h in case of administrative disasters, but cannot afford to store them for much longer because you might have high turnover volume in the database. On the receiving side, you may have more disk space available, or need to comply with other backup retention policies.

zrepl uses a set of keep rules to determine which snapshots shall be kept per filesystem. A snapshot that is not kept by any rule is destroyed. The keep rules are evaluated on the active side (push or pull job) of the replication setup, for both active and passive side, after replication completed or was determined to have failed permanently.

Example Configuration:

jobs:
  - type: push
    name: ...
    connect: ...
    filesystems: {
      "<": true,
      "tmp": false
    }
    snapshotting:
      type: periodic
      prefix: zrepl_
      interval: 10m
    pruning:
      keep_sender:
        - type: not_replicated
        # make sure manually created snapshots by the administrator are kept
        - type: regex
          regex: "^manual_.*"
        - type: grid
          grid: 1x1h(keep=all) | 24x1h | 14x1d
          regex: "^zrepl_.*"
      keep_receiver:
        - type: grid
          grid: 1x1h(keep=all) | 24x1h | 35x1d | 6x30d
          regex: "^zrepl_.*"
        # manually created snapshots will be kept forever on receiver

Danger

You might have existing snapshots of filesystems affected by pruning which you want to keep, i.e. not be destroyed by zrepl. Make sure to actually add the necessary regex keep rules on both sides, like with manual in the example above.

Attention

It is currently not possible to define pruning on a source job. The source job creates snapshots, which means that extended replication downtime will fill up the source’s zpool with snapshots, since pruning is directed by the corresponding active side (pull job). If this is a potential risk for you, consider using push mode.

Policy not_replicated

jobs:
- type: push
  pruning:
    keep_sender:
    - type: not_replicated
  ...

not_replicated keeps all snapshots that have not been replicated to the receiving side. It only makes sense to specify this rule on a sender (source or push job). The state required to evaluate this rule is stored in the replication cursor bookmark on the sending side.

Policy grid

jobs:
- type: pull
  pruning:
    keep_receiver:
    - type: grid
      regex: "^zrepl_.*"
      grid: 1x1h(keep=all) | 24x1h | 35x1d | 6x30d
            │                │
            └─ one hour interval
                             │
                             └─ 24 adjacent one-hour intervals
  ...

The retention grid can be thought of as a time-based sieve: The grid field specifies a list of adjacent time intervals: the left edge of the leftmost (first) interval is the creation date of the youngest snapshot. All intervals to its right describe time intervals further in the past.

Each interval carries a maximum number of snapshots to keep. It is specified via (keep=N), where N is either all (all snapshots are kept) or a positive integer. The default value is keep=1.

The following procedure happens during pruning:

  1. The list of snapshots is filtered by the regular expression in regex. Only snapshots names that match the regex are considered for this rule, all others are not affected.
  2. The filtered list of snapshots is sorted by creation
  3. The left edge of the first interval is aligned to the creation date of the youngest snapshot
  4. A list of buckets is created, one for each interval
  5. The list of snapshots is split up into the buckets.
  6. For each bucket
    1. the contained snapshot list is sorted by creation.
    2. snapshots from the list, oldest first, are destroyed until the specified keep count is reached.
    3. all remaining snapshots on the list are kept.

Policy last_n

jobs:
  - type: push
    pruning:
      keep_receiver:
      - type: last_n
        count: 10
  ...

last_n keeps the last count snapshots (last = youngest = most recent creation date).

Policy regex

jobs:
  - type: push
    pruning:
      keep_receiver:
      - type: regex
        regex: "^(zrepl|manual)_.*"
 ...

regex keeps all snapshots whose names are matched by the regular expressionin regex. Like all other regular expression fields in prune policies, zrepl uses Go’s regexp.Regexp Perl-compatible regular expressions (Syntax).