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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
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