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README.rst

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):

$ 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:

-- 90 % of free memory after machine restart
cache.size = 13453 * MB

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.

# /etc/fstab
tmpfs        /var/cache/knot-resolver        tmpfs   rw,size=2G,uid=knot-resolver,gid=knot-resolver,nosuid,nodev,noexec,mode=0700 0 0

# /etc/knot-resolver/config
cache.size = 2 * GB