Understanding cache utilisation

Guillaume Quintard guillaume at varnish-software.com
Tue Nov 7 07:09:06 UTC 2017

Hello Jason,

Just an idea, before patching: log obj.hits and obj.age to get an idea of
what you are delivering.

Guillaume Quintard

On Nov 7, 2017 07:23, "Jason Stangroome" <jason at codeassassin.com> wrote:

> Hello,
> For at least one site that I am using Varnish Cache for, the allocated
> cache size is fully utilised and objects are now being nuked/evicted
> from the cache. Before I blindly supply more resources to provide a
> larger cache, I'd like to understand if the cache space is being used
> optimally.
> For example, of the objects that are expiring naturally, were they
> ever used to serve a cache hit? If not, it was probably wasteful to
> store them in the cache in the first place.
> Of the objects that are being nuked/evicted, were they close to their
> natural expiration time anyway? Were they ever used to serve a cache
> hit?
> Of the objects that have been nuked, are they requested shortly after,
> suggesting it may have been worth having a larger cache in order to
> retain them a bit longer?
> I did not see an easy way to answer these questions with the standard
> Varnish toolset - am I missing something?
> I looked through the code paths in Varnish 5.1 responsible for
> expiration and nuking and it seems the object hit count and ttl is
> available in that context - would it be worth writing a patch to log
> these fields to the VSL?
> How do others ensure their allocated cache size is well utilised
> before growing the cache?
> Regards,
> Jason
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