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 per sending and receiving side 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.

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:
      # keep all snapshots with prefix zrepl_ or manual_
      - type: regex
        regex: "^(zrepl|manual)_.*"

  - type: push
    snapshotting:
      prefix: zrepl_
    pruning:
      keep_sender:
      # keep all snapshots that were not created by zrepl
      - type: regex
        negate: true
        regex: "^zrepl_.*"

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). The optional negate boolean field inverts the semantics: Use it if you want to keep all snapshots that do not match the given regex.

Source-side snapshot pruning

A source jobs takes snapshots on the system it runs on. The corresponding pull job on the replication target connects to the source job and replicates the snapshots. Afterwards, the pull job coordinates pruning on both sender (the source job side) and receiver (the pull job side).

There is no built-in way to define and execute pruning on the source side independently of the pull side. The source job will continue taking snapshots which will not be pruned until the pull side connects. This means that extended replication downtime will fill up the source’s zpool with snapshots.

If the above is a conceivable situation for you, consider using push mode, where pruning happens on the same side where snapshots are taken.

Workaround using snap job

As a workaround (see GitHub issue #102 for development progress), a pruning-only snap job can be defined on the source side: The snap job is in charge of snapshot creation & destruction, whereas the source job’s role is reduced to just serving snapshots. However, since, jobs are run independently, it is possible that the snap job will prune snapshots that are queued for replication / destruction by the remote pull job that connects to the source job. Symptoms of such race conditions are spurious replication and destroy errors.

Example configuration:

# source side
jobs:
- type: snap
  snapshotting:
    type: periodic
  pruning:
    keep:
      # source side pruning rules go here
  ...

- type: source
  snapshotting:
    type: manual
  root_fs: ...

# pull side
jobs:
- type: pull
  pruning:
    keep_sender:
      # let the source-side snap job do the pruning
      - type: regex
        regex: ".*"
      ...
    keep_receiver:
      # feel free to prune on the pull side as desired
      ...