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75 lines
4.2 KiB
ReStructuredText
75 lines
4.2 KiB
ReStructuredText
.. include:: ../global.rst.inc
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Replication Options
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===================
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::
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jobs:
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- type: push
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filesystems: ...
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replication:
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protection:
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initial: guarantee_resumability # guarantee_{resumability,incremental,nothing}
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incremental: guarantee_resumability # guarantee_{resumability,incremental,nothing}
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concurrency:
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size_estimates: 4
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steps: 1
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...
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.. _replication-option-protection:
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``protection`` option
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--------------------------
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The ``protection`` variable controls the degree to which a replicated filesystem is protected from getting out of sync through a zrepl pruner or external tools that destroy snapshots.
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zrepl can guarantee :ref:`resumability <step-holds>` or just :ref:`incremental replication <replication-cursor-and-last-received-hold>`.
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``guarantee_resumability`` is the **default** value and guarantees that a replication step is always resumable and that incremental replication will always be possible.
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The implementation uses replication cursors, last-received-hold and step holds.
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``guarantee_incremental`` only guarantees that incremental replication will always be possible.
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If a step ``from -> to`` is interrupted and its `to` snapshot is destroyed, zrepl will remove the half-received ``to``'s resume state and start a new step ``from -> to2``.
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The implementation uses replication cursors, tentative replication cursors and last-received-hold.
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``guarantee_nothing`` does not make any guarantees with regards to keeping sending and receiving side in sync.
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No bookmarks or holds are created to protect sender and receiver from diverging.
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**Tradeoffs**
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Using ``guarantee_incremental`` instead of ``guarantee_resumability`` obviously removes the resumability guarantee.
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This means that replication progress is no longer monotonic which might lead to a replication setup that never makes progress if mid-step interruptions are too frequent (e.g. frequent network outages).
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However, the advantage and :issue:`reason for existence <288>` of the ``incremental`` mode is that it allows the pruner to delete snapshots of interrupted replication steps
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which is useful if replication happens so rarely (or fails so frequently) that the amount of disk space exclusively referenced by the step's snapshots becomes intolerable.
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.. NOTE::
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When changing this flag, obsoleted zrepl-managed bookmarks and holds will be destroyed on the next replication step that is attempted for each filesystem.
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.. _replication-option-concurrency:
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``concurrency`` option
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----------------------
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The ``concurrency`` options control the maximum amount of concurrency during replication.
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The default values allow some concurrency during size estimation but no parallelism for the actual replication.
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* ``concurrency.steps`` (default = 1) controls the maximum number of concurrently executed :ref:`replication steps <overview-how-replication-works>`.
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The planning step for each file system is counted as a single step.
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* ``concurrency.size_estimates`` (default = 4) controls the maximum number of concurrent step size estimations done by the job.
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Note that initial replication cannot start replicating child filesystems before the parent filesystem's initial replication step has completed.
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Some notes on tuning these values:
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* Disk: Size estimation is less I/O intensive than step execution because it does not need to access the data blocks.
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* CPU: Size estimation is usually a dense CPU burst whereas step execution CPU utilization is stretched out over time because of disk IO.
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Faster disks, sending a compressed dataset in :ref:`plain mode <zfs-background-knowledge-plain-vs-raw-sends>` and the zrepl transport mode all contribute to higher CPU requirements.
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* Network bandwidth: Size estimation does not consume meaningful amounts of bandwidth, step execution does.
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* :ref:`zrepl ZFS abstractions <zrepl-zfs-abstractions>`: for each replication step zrepl needs to update its ZFS abstractions through the ``zfs`` command which often waits multiple seconds for the zpool to sync.
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Thus, if the actual send & recv time of a step is small compared to the time spent on zrepl ZFS abstractions then increasing step execution concurrency will result in a lower overall turnaround time.
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