Query strings and cache

[Paul Prescod <pa...@prescod.net>]

This document: http://httpd.apache.org/docs/2.2/mod/mod_cache.html

Says: "Ordinarily, requests with query string parameters are cached
separately for each unique query string"

This document: http://httpd.apache.org/docs/2.2/caching.html

Says: " If the URL included a query string (e.g. from a HTML form GET
method) it will not be cached unless the response includes an
"Expires:" header, as per RFC2616 section 13.9."

And the former document is a bit confusing. Under the title "What Can
Be Cached" it talks a lot about headers but does not discuss the use
of a memory-based cache in front of a simple file system. It implies
that the cache is always running in front of some other process that
is setting headers.

Another thing that confuses me is that the documentation talks in
various places about "the file descriptor cache mode in mod_mem_cache"
but I can't find an actual description of it.  The main documentation
for mod_mem_cache is not at all clear what it means to "cache open
file descriptors" and when it is better or worse than caching in the
heap.

Should I infer that the "fd" mode is equivalent to the CacheFile
capability of mod_file_cache ?

In addition, two questions that are not directly documentation-related
questions:

 1. How can I monitor what Apache's cache is doing? How do I know what
it is caching?

 2. If my site is occasionally serving tiny graphic and CSS files
slowly, where should I look? iostat says that my machine is 95% idle,
iowait is less than 1 %, cached memory is measured in gigabytes, as is
buffered memory. It's ridiculous that I frequently see small files in
my logs that take anywhere from 15 to 30 seconds to serve. I'd like to
get memory caching working but really I shouldn't need it!

 Paul Prescod

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