== Preparing a high load web mirror setup

I spent a chunk of this weekend preparing a mirror for a high load
environment. The mirror only needs to serve a couple of [[large
video clips http://themis.asu.edu/valles_video]], but they're
{{ST:strike:going to be}} linked from a [[high traffic website
http://www.google.com/mars/]], so we expect a lot of (simultaneous)
connections and a lot of outgoing bandwidth.

I made a generic mirror url, using a new hostname that I had to put into
a [[new sub-zone in our DNS ADNSRealization]]. Right now it has two _A_
records, each with a five minute TTL. Each is on a different 100 Mbit/s
subnet; because our subnet uplinks to the university backbone are only
100 mbit, this is the only way I can do over 100 mbit/s outbound.

The machines run [[lighttpd http://www.lighttpd.net/]], serving just the
mirrored files, with enough memory to keep the files in cache. Lighttpd
is small and easy to install (nice when you're in a rush), plus as a
single process server without threads or forking it can't really kill a
machine no matter how many simultaneous connections it gets. (I chose
[[lighttpd]] over [[thttpd http://www.acme.com/software/thttpd/]] for
reasons I may go into later.)

Looking at this after writing it up, it's surprising to me how little
stuff is actually involved. Hopefully this setup will work fine in
practice; I'll likely find out soon.

(The setup passed stress tests, but that's not the same as having
real load show up.)

=== Sidebar: lighttpd configuration

Lighttpd has a helpful web page on [[performance improvements
http://www.lighttpd.net/documentation/performance.html]].
I turned keep-alives off; as far as I can see, keep-alives
are useless for serving unrelated static files and I wanted
to maximize how many simultaneous connections I could handle.

In testing, I discovered I had to increase [[lighttpd]]'s
_server.max-fds_ parameter from 1024. A little thought led me to a doh
moment about this: of course I needed to bump it, because every incoming
connection needs *two* file descriptors; one for the network socket and
one for the file being sent out. So with 1024 file descriptors the web
server could only handle about 500 simultaneous connections.