There appears to be a bug in 0.7.55 (as well as 0.7.54, and perhaps others) when serving .flv files.

Connections never free. If you look at the stub_status page, the list of writers increases without limit. On a completely idle machine, connections that use the flv module are permanently added to the "writing" list, no matter how long it has been since the connection was closed; connections that do not use the flv module increment the other connection counters, as expected, but do not create lingering "ghost writers."

The server in question is hosting the flv files from an XFS partition. The nginx executable has been linked with libhugepages and is fully loaded in a tlbfs segment. Sendfile is enabled and it makes no difference what the output buffers parameter is set to. I have tried it with directio on and off with the same result: with directio on, nginx leaks buffer memory, increasing the size of its memory image without limit; with directio off, it would (eventually, if I let it) exhaust file descriptors.

This problem does not exist for 0.6.36 using the exact same compilation options, configuration files and spool directory.