maximum number of seconds until a waiting write call times out and closes the connection
If you get warnings like this in the error log:
'NOTE: a request for ... timed out after writing xxxxx bytes. We waited 360 seconds. If this a problem increase server.max-write-idle'
It's most likely caused by slow client connections. Apparently a client requested a file and lighttpd did not complete serving this file. This happens when the client's connection drops or the client is really slow. In this case there's no real need to ajust the server.max-write-idle option.