doc/user/deployment-docker.rst: added info about cache

diff --git a/doc/user/deployment-docker.rst b/doc/user/deployment-docker.rst
index 0ddf0447c7d6d80ec8511b465110a3a677d40fd7..42f5503b3db88db72cf57ba8ec202d88b768937a 100644 (file)
--- a/doc/user/deployment-docker.rst
+++ b/doc/user/deployment-docker.rst
@@ -26,6 +26,18 @@ Now you can try sending a query to the resolver using `kdig <https://www.knot-dn
 
 The image contains full Knot Resolver installation, so there shouldn't be much difference between running it natively and running it in a container.
 The configuration file is located at ``/etc/knot-resolver/config.yaml`` and the cache is at ``/var/cache/knot-resolver``.
+Please do not change these and other paths (rundir) as you may break things inside the container.
+
+.. warning::
+    
+    Beware of running the container with a software defined network (i.e. in Kubernetes).
+    This will likely to result in some performance losses.
+    We haven't done any measurements comparing different types of installations so we don't know the performance differences.
+    If you have done your own measurements yourself, please contact us and we will share it with everyone else.
+
+------
+Config
+------
 
 We recommend persistent configuration across container restarts,
 for more see `Docker persisting container data <https://docs.docker.com/get-started/docker-concepts/running-containers/persisting-container-data/>`_.
@@ -42,9 +54,15 @@ Get ``CONTAINER_ID`` using the ``docker ps`` command or give your container name
 
     $ docker exec -it CONTANER_ID kresctl reload
 
-.. warning::
-    
-    Beware of running the container with a software defined network (i.e. in Kubernetes).
-    This will likely to result in some performance losses.
-    We haven't done any measurements comparing different types of installations so we don't know the performance differences.
-    If you have done your own measurements yourself, please contact us and we will share it with everyone else.
+-----
+Cache
+-----
+
+You can also use persistent volume for the cache, but sharing it between more containers requires special option ``--pid=host``.
+This is caused by the LMDB's reliance on unique PID numbers, see `issue #637 <https://gitlab.nic.cz/knot/knot-resolver/-/issues/637>`_ for more information.
+It is also good to mount the cache on ``tmpfs`` (semi-persistent), otherwise it will not work well under heavy load.
+
+.. code-block:: bash
+
+    $ docker volume create --opt type=tmpfs --opt device=tmpfs cache
+    $ docker run --rm -ti --pid=host --network host -v cache:/var/cache/knot-resolver docker.io/cznic/knot-resolver:6