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Re: cache hit rate isn't what I'd expect

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On 29/09/17 11:29, Aaron Turner wrote:
So this grep through my access logs for this single URL does a good
job illustrating a rather interesting problem:

$ grep -h 'https://static.licdn.com/sc/h/ddzuq7qeny6qn0ysh3hj6pzmr
text/css ip_index=0,client=m0078269' access.*.log | sort


...
>
At first I thought this was because the because I have a bunch of
clients, each of which behaves exactly the same except for one thing:
the client includes a unique request header that squid strips off
before forwarding to the server (you can see it logged as
client=mXXXXX_XXXX).  But in this case I've controlled for that and
only grep'd for a single client's request.  I've even tried setting
"vary_ignore_expire on", but that doesn't seem to be a complete fix.

I can't for the life of me understand why the low hit rate though.


The duration and size fields are quite useful for detecting reasons for HIT/MISS.

Request headers should not affect the response caching, unless they are listed in the servers Vary header.

In this case the server is delivering broken Vary responses. redbot.org says it is using Vary:Accept-Encoding sometimes, so both the Vary and Accept-Encoding would be useful info to log.

I expect it is the usual problem of clients fighting over whose variant gets cached when this type of server breakage happens - when the Vary header changes or disappears, old variants become unfindable until it changes back.

Amos
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