Warning: This document is for an old version of zrepl. The main version is master.

Pruning Policies

In zrepl, pruning means destroying snapshots by some policy.

A pruning policy takes a list of snapshots and – for each snapshot – decides whether it should be kept or destroyed.

The job context defines which snapshots are even considered for pruning, for example through the snapshot_prefix variable. Check the respective job definition for details.

Currently, the Retention Grid is the only supported pruning policy.

Retention Grid

jobs:
- name: pull_app-srv
  ...
  prune:
    policy: grid
    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 secified via (keep=N), where N is either all (all snapshots are kept) or a positive integer. The default value is 1.

The following procedure happens during pruning:

  1. The list of snapshots eligible for pruning is sorted by creation
  2. The left edge of the first interval is aligned to the creation date of the youngest snapshot
  3. A list of buckets is created, one for each interval
  4. The list of snapshots is split up into the buckets.
  5. 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.

Attention

The configuration of the first interval (1x1h(keep=all) in the example) determines the maximum allowable replication lag between source and destination. After the first interval, source and destination likely have different retention settings. This means source and destination may prune different snapshots, prohibiting incremental replication froms snapshots that are not in the first interval.

Always configure the first interval to 1x?(keep=all), substituting ? with the maximum time replication may fail due to downtimes, maintenance, connectivity issues, etc. After outages longer than ? you may be required to perform full replication again.