Already downloaded files are not cached by trafficserver 5.x (caching forward proxy)?

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http://askubuntu.com – I want to setup Apache trafficserver 5.1.0 installed from source on Ubuntu 14.04 to use a large amount of disk space to preserve large download files as well as web sites (in order to get started I don't care so much if anything else is preserved, the eviction policy should take care about that anyway). I don't get anything from the logs, so I'm controlling traffic with nethogs which shows that the files which are requested by wget are downloaded by the trafficserver process, but the download occurs at every request, i.e. the cache works, but not the caching like I want it to. My test case is (HowTos)