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68 lines
2.8 KiB
ReStructuredText
68 lines
2.8 KiB
ReStructuredText
.. _prune:
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Pruning Policies
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================
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In zrepl, *pruning* means *destroying snapshots by some policy*.
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A *pruning policy* takes a list of snapshots and -- for each snapshot -- decides whether it should be kept or destroyed.
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The job context defines which snapshots are even considered for pruning, for example through the ``snapshot_prefix`` variable.
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Check the respective :ref:`job definition <job>` for details.
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Currently, the :ref:`prune-retention-grid` is the only supported pruning policy.
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.. TIP::
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You can perform a dry-run of a job's pruning policy using the ``zrepl test`` subcommand.
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.. _prune-retention-grid:
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Retention Grid
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--------------
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::
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jobs:
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- name: pull_app-srv
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...
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prune:
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policy: grid
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grid: 1x1h(keep=all) | 24x1h | 35x1d | 6x30d
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│ │
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└─ one hour interval
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│
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└─ 24 adjacent one-hour intervals
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The retention grid can be thought of as a time-based sieve:
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The ``grid`` field specifies a list of adjacent time intervals:
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the left edge of the leftmost (first) interval is the ``creation`` date of the youngest snapshot.
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All intervals to its right describe time intervals further in the past.
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Each interval carries a maximum number of snapshots to keep.
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It is secified via ``(keep=N)``, where ``N`` is either ``all`` (all snapshots are kept) or a positive integer.
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The default value is **1**.
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The following procedure happens during pruning:
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#. The list of snapshots eligible for pruning is sorted by ``creation``
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#. The left edge of the first interval is aligned to the ``creation`` date of the youngest snapshot
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#. A list of buckets is created, one for each interval
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#. The list of snapshots is split up into the buckets.
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#. For each bucket
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#. the contained snapshot list is sorted by creation.
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#. snapshots from the list, oldest first, are destroyed until the specified ``keep`` count is reached.
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#. all remaining snapshots on the list are kept.
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.. ATTENTION::
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The configuration of the first interval (``1x1h(keep=all)`` in the example) determines the **maximum allowable replication lag** because the source and destination pruning policies do not coordinate:
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if replication does not work for whatever reason, source will continue to execute the prune policy.
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Eventually, source destroys a snapshot that has never been replicated to destination, degrading the temporal resolution of your backup.
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Thus, **always** configure the first interval to ``1x?(keep=all)``, substituting ``?`` with the maximum time replication may fail due to downtimes, maintenance, connectivity issues, etc.
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.. We intentionally do not mention that bookmarks are used to bridge the gap between source and dest that are out of sync snapshot-wise. This is an implementation detail.
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