Adding upstream version 5.6.0.upstream/5.6.0upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 69 insertions, 0 deletions

diff --git a/lib/cache/README.rst b/lib/cache/README.rst
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+.. SPDX-License-Identifier: GPL-3.0-or-later
+
+.. _cache_sizing:
+
+Cache sizing
+------------
+
+For personal use-cases and small deployments cache size around 100 MB is more than enough.
+
+For large deployments we recommend to run Knot Resolver on a dedicated machine, and to allocate 90% of machine's free memory for resolver's cache.
+
+For example, imagine you have a machine with 16 GB of memory.
+After machine restart you use command ``free -m`` to determine amount of free memory (without swap):
+
+.. code-block:: bash
+
+  $ free -m
+                total        used        free
+  Mem:          15907         979       14928
+
+Now you can configure cache size to be 90% of the free memory 14 928 MB, i.e. 13 453 MB:
+
+.. code-block:: lua
+
+   -- 90 % of free memory after machine restart
+   cache.size = 13453 * MB
+
+.. _cache_persistence:
+
+Cache persistence
+-----------------
+.. tip:: Using tmpfs for cache improves performance and reduces disk I/O.
+
+By default the cache is saved on a persistent storage device
+so the content of the cache is persisted during system reboot.
+This usually leads to smaller latency after restart etc.,
+however in certain situations a non-persistent cache storage might be preferred, e.g.:
+
+  - Resolver handles high volume of queries and I/O performance to disk is too low.
+  - Threat model includes attacker getting access to disk content in power-off state.
+  - Disk has limited number of writes (e.g. flash memory in routers).
+
+If non-persistent cache is desired configure cache directory to be on
+tmpfs_ filesystem, a temporary in-memory file storage.
+The cache content will be saved in memory, and thus have faster access
+and will be lost on power-off or reboot.
+
+
+.. note:: In most of the Unix-like systems ``/tmp`` and ``/var/run`` are commonly mounted to tmpfs.
+   While it is technically possible to move the cache to an existing
+   tmpfs filesystem, it is *not recommended*: The path to cache is specified in
+   multiple systemd units, and a shared tmpfs space could be used up by other
+   applications, leading to ``SIGBUS`` errors during runtime.
+
+Mounting the cache directory as tmpfs_ is recommended approach.
+Make sure to use appropriate ``size=`` option and don't forget to adjust the
+size in the config file as well.
+
+.. code-block::
+
+   # /etc/fstab
+   tmpfs	/var/cache/knot-resolver	tmpfs	rw,size=2G,uid=knot-resolver,gid=knot-resolver,nosuid,nodev,noexec,mode=0700 0 0
+
+.. code-block:: lua
+
+   # /etc/knot-resolver/config
+   cache.size = 2 * GB
+
+.. _tmpfs: https://en.wikipedia.org/wiki/Tmpfs