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