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docs: update multi-job & multi-host setup section

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Christian Schwarz 2020-09-05 17:45:18 +02:00
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docs/configuration/overview.rst
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@ -80,16 +80,19 @@ The active side (:ref:`push <job-push>` and :ref:`pull <job-pull>` job) executes
.. TIP::
The progress of the active side can be watched live using the ``zrepl status`` subcommand.
.. _overview-passive-side--client-identity:
How the Passive Side Works
--------------------------
The passive side (:ref:`sink <job-sink>` and :ref:`source <job-source>`) waits for connections from the corresponding active side,
using the transport listener type specified in the ``serve`` field of the job configuration.
Each transport listener provides a client's identity to the passive side job.
It uses the client identity for access control:
When a client connects, the transport listener performS listener-specific access control (cert validation, IP ACLs, etc)
and determines the *client identity*.
The passive side job then uses this client identity as follows:
* The ``sink`` job maps requests from different client identities to their respective sub-filesystem tree ``root_fs/${client_identity}``.
* The ``source`` job has a whitelist of client identities that are allowed pull access.
* The ``source`` might, in the future, embed the client identity in :ref:`zrepl's ZFS abstraction names <zrepl-zfs-abstractions>` in order to support multi-host replication.
.. TIP::
The implementation of the ``sink`` job requires that the connecting client identities be a valid ZFS filesystem name components.
@ -164,7 +167,7 @@ With the background knowledge from the previous paragraph, we now summarize the
.. _replication-placeholder-property:
**Placeholder filesystems** on the receiving side are regular ZFS filesystems with the placeholder property ``zrepl:placeholder=on``.
**Placeholder filesystems** on the receiving side are regular ZFS filesystems with the ZFS property ``zrepl:placeholder=on``.
Placeholders allow the receiving side to mirror the sender's ZFS dataset hierarchy without replicating every filesystem at every intermediary dataset path component.
Consider the following example: ``S/H/J`` shall be replicated to ``R/sink/job/S/H/J``, but neither ``S/H`` nor ``S`` shall be replicated.
ZFS requires the existence of ``R/sink/job/S`` and ``R/sink/job/S/H`` in order to receive into ``R/sink/job/S/H/J``.
@ -229,39 +232,53 @@ Limitations
Multiple Jobs & More than 2 Machines
------------------------------------
The quick-start guides focus on simple setups with a single sender and a single receiver.
This section documents considerations for more complex setups.
.. ATTENTION::
When using multiple jobs across single or multiple machines, the following rules are critical to avoid race conditions & data loss:
Before you continue, make sure you have a working understanding of :ref:`how zrepl works <overview-how-replication-works>`
and :ref:`what zrepl does to ensure <zrepl-zfs-abstractions>` that replication between sender and receiver is always
possible without conflicts.
This will help you understand why certain kinds of multi-machine setups do not (yet) work.
1. The sets of ZFS filesystems matched by the ``filesystems`` filter fields must be disjoint across all jobs configured on a machine.
2. The ZFS filesystem subtrees of jobs with ``root_fs`` must be disjoint.
3. Across all zrepl instances on all machines in the replication domain, there must be a 1:1 correspondence between active and passive jobs.
.. NOTE::
Explanations & exceptions to above rules are detailed below.
If you can't find your desired configuration, have questions or would like to see improvements to multi-job setups, please `open an issue on GitHub <https://github.com/zrepl/zrepl/issues/new>`_.
If you would like to see improvements to multi-job setups, please `open an issue on GitHub <https://github.com/zrepl/zrepl/issues/new>`_.
Multiple Jobs on one Machine
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
As a general rule, multiple jobs configured on one machine **must operate on disjoint sets of filesystems**.
Otherwise, concurrently running jobs might interfere when operating on the same filesystem.
No Overlapping
^^^^^^^^^^^^^^
On your setup, ensure that
Jobs run independently of each other.
If two jobs match the same filesystem with their ``filesystems`` filter, they will operate on that filesystem independently and potentially in parallel.
For example, if job A prunes snapshots that job B is planning to replicate, the replication will fail because B assumed the snapshot to still be present.
However, the next replication attempt will re-examine the situation from scratch and should work.
* all ``filesystems`` filter specifications are disjoint
* no ``root_fs`` is a prefix or equal to another ``root_fs``
* no ``filesystems`` filter maches any ``root_fs``
N push jobs to 1 sink
^^^^^^^^^^^^^^^^^^^^^
**Exceptions to the rule**:
The :ref:`sink job <job-sink>` namespaces by client identity.
It is thus safe to push to one sink job with different client identities.
If the push jobs have the same client identity, the filesystems matched by the push jobs must be disjoint to avoid races.
* A ``snap`` and ``push`` job on the same machine can match the same ``filesystems``.
To avoid interference, only one of the jobs should be pruning snapshots on the sender, the other one should keep all snapshots.
Since the jobs won't coordinate, errors in the log are to be expected, but :ref:`zrepl's ZFS abstractions <zrepl-zfs-abstractions>` ensure that ``push`` and ``sink`` can always replicate incrementally.
This scenario is detailed in one of the :ref:`quick-start guides <quickstart-backup-to-external-disk>`.
N pull jobs from 1 source
^^^^^^^^^^^^^^^^^^^^^^^^^
Multiple pull jobs pulling from the same source have potential for race conditions during pruning:
each pull job prunes the source side independently, causing replication-prune and prune-prune races.
More Than 2 Machines
^^^^^^^^^^^^^^^^^^^^
There is currently no way for a pull job to filter which snapshots it should attempt to replicate.
Thus, it is not possible to just manually assert that the prune rules of all pull jobs are disjoint to avoid replication-prune and prune-prune races.
This section might be relevant to users who wish to *fan-in* (N machines replicate to 1) or *fan-out* (replicate 1 machine to N machines).
**Working setups**:
* N ``push`` identities, 1 ``sink`` (as long as the different push jobs have a different :ref:`client identity <overview-passive-side--client-identity>`)
* ``sink`` constrains each client to a disjoint sub-tree of the sink-side dataset hierarchy ``${root_fs}/${client_identity}``.
Therefore, the different clients cannot interfere.
**Setups that do not work**:
* N ``pull`` identities, 1 ``source`` job. Tracking :issue:`380`.