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Cedric H.
Asked: 2016-10-03 10:06:59 +0800 CST

Docker swarm and volumes

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Considering Docker 1.12. I understand that volumes are not per say portable across the swarm and that solutions like Flocker would be better for larger clusters.

Nevertheless, is the option I describe below acceptable for my setup?

Setup A swarm with 3 nodes runnin Docker 1.12.latest from the experimental branch.

Goal Run a nice but small Cassandra cluster.

Problem I want to decouple the stateful part of the service (the data volume) from the stateless Cassandra daemons.

Proposed solution Create a named volume manually on each node with the same name on each node. Use docker service create --name cassandra --replicas 3 --mount type=volume,source=cassandra_data,target=/usr/var/cassandra

My understanding is that one replica of the Cassandra will be run and every node, each one referring a local named volume with the same name.

Would this work? Do I need to specify a constraint to make sure there is at max 1 replica per node of the swarm?

Edit: reading the doc, it seems I should use --mode=global instead of replicas=3 to run exactly 1 replica per node of the swarm. Which makes a lot of sense...

docker docker-swarm cassandra

2 Answers

  1. Yes this does work, and using the global flag will be a better option to prevent any conflicts. I have run a similar model to this with MongoDB as a test and worked perfectly.

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  2. As of the 19.03 release, there is now an option to limit the number of replicas per node to avoid multiple replicas accessing the same volume:

    $ docker service create --help

Usage:  docker service create [OPTIONS] IMAGE [COMMAND] [ARG...]

Create a new service

Options:
...
      --replicas-max-per-node uint         Maximum number of tasks per node (default 0 = unlimited)

    With this added to the service create command, the next step will be adding a placement preference in the compose file to support docker stack deploy.

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