Monitoring Varnish Cache Hit Rate (Ratio) with check_varnish

Written by Claudio Kuenzler - 0 comments

Published on January 9th 2023 - Listed in Varnish Monitoring

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Personally I have been using check_varnish.py to monitor Varnish Cache (and Plus) for a long time. In the past I've also contributed a couple of times in this open source monitoring plugin and described how to use check_varnish and create graphs from Varnish statistics.

The Cache Hit Rate

As I was implementing a different cache/memory backend, I wanted to compare the Cache Hit Rate before and after the cache changes. The Cache Hit Rate (sometimes also named Cache Hit Ratio) is a very important value telling you how many of your requests are served from the cache. As Arianna Aondio describes it:

The cache hit rate is the percentage of requests that result in cache hits. In Varnish it is evaluated as cache_hit / (cache_hit + cache_miss). The higher the hit rate the more effective your cache is.

However varnishstat does not show this object. It needs to be calculcated manually using the MAIN.cache_hit and MAIN.cache_miss values.

root@varnish:~# varnishstat -n varnish3 -1 | grep MAIN.cache
MAIN.cache_hit                  43616371        27.82 Cache hits
MAIN.cache_hit_grace            14902812         9.51 Cache grace hits
MAIN.cache_hitpass                     0         0.00 Cache hits for pass.
MAIN.cache_hitmiss               2043449         1.30 Cache hits for miss.
MAIN.cache_miss                 32955140        21.02 Cache misses

Using these values and the formula mentioned above this results in:

Cache Hit Rate = cache_hit / (cache_hit + cache_miss)
therefore
0.569616172 = 43616371 / (43616371 + 32955140)

Adding Cache Hit Rate calculation in check_varnish

Of course this can be automated. We already have the check_varnish monitoring plugin doing most of the work. It just needs to obtain the values of MAIN.cache_hit and MAIN.cache_miss and apply the formula. Voilà, the Cache Hit Rate value is there.

The next version (most likely 1.5) of check_varnish will be able to calculate the Cache Hit Rate by applying the -r or --hitratio parameter to the plugin:

root@varnish:~# /usr/lib/nagios/plugins/check_varnish.py -n varnish3 -f MAIN.cache_hit,MAIN.cache_miss -r
VARNISH OK - MAIN.cache_hit is 43629247 - MAIN.cache_miss is 32964372 - Cache Hit Rate is 0.57 -  | MAIN.cache_hit=43629247;0;0;; MAIN.cache_miss=32964372;0;0;; hitrate=0.57;0;0;;

Requirement for this is that both MAIN.cache_hit and MAIN.cache_miss are mentioned in the fields (-f) list. The plugin will exit with a state unknown in this (human) error is made:

root@varnish:~# /usr/lib/nagios/plugins/check_varnish.py -n varnish3 -f MAIN.backend_fail -r
VARNISH UNKNOWN - Must use MAIN.cache_hit and MAIN.cache_miss in field list to calculate hit ratio

The pull request with the code changes was sent in today.

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