My Goal: Let nscd maintain a fairly large DNS cache in excess memory since I have it available.
I have a webserver that has a broadly dispersed but high-repeat user base. It has plenty of memory so I thought I'd improve response time by caching lookups but according to
nscd -g I'm only at a 6% cache hit rate (meaning
nscd is most likely introducing more latency saving to the cache or looking through the cache for an entry it will never find, than it's preventing by going out to the network):
hosts cache: yes cache is enabled yes cache is persistent yes cache is shared 211 suggested size 216064 total data pool size 2328 used data pool size 36000 seconds time to live for positive entries 20 seconds time to live for negative entries 4455 cache hits on positive entries 0 cache hits on negative entries 17357 cache misses on positive entries 42348 cache misses on negative entries 6% cache hit rate 17 current number of cached values 40 maximum number of cached values 3 maximum chain length searched 0 number of delays on rdlock 0 number of delays on wrlock 0 memory allocations failed yes check /etc/hosts for changes
Probably a large contributor to the 6% hit rate is the fact that apparently it's only cached 17 entries. Doing a
strings /var/db/nscd/hosts shows that the host cache entries it has created are mostly for machines on our internal network. It's good to have these cached since the daily re-publish of the website is likely sped up but my goal is to speed up end user experience without making any real configuration changes.
This is the relevant segment of
threads 10 server-user nscd debug-level 0 paranoia no [.....snip......] enable-cache hosts yes positive-time-to-live hosts 36000 negative-time-to-live hosts 20 suggested-size hosts 10657 check-files hosts yes persistent hosts yes shared hosts yes max-db-size hosts 33554432
Basically, I need help understanding how my host cache can be so small even though I've set the positive TTL's on the host cache to be incredibly high. I'm sure it's the small number of actual cached entries that is causing the hit rate to be so low.
I'm assuming since the hit rate is 6% but my positive TTL is fairly large, that means my current workload is performing DNS host lookups, but they're just not being save. I have no idea why these aren't being saved nor what to check next. What I had expected would be a fairly large DNS cache now.
Even if the hit rate stayed small (i.e: clients weren't repeating as often as I thought) I'd still expect those DNS lookups to be cached but looking at the "current number of cached values" that doesn't appear to be happening either.