This flag makes sub filter flush buffered data and optimizes allocation in copy
filter.

With main request buffered, it's possible, that a slice subrequest will send
output before it.  For example, while main request is waiting for aio read to
complete, a slice subrequest can start an aio operation as well.  The order
in which aio callbacks are called is undetermined.

Splits a request into subrequests, each providing a specific range of response.
The variable "$slice_range" must be used to set subrequest range and proper
cache key.  The directive "slice" sets slice size.

The following example splits requests into 1-megabyte cacheable subrequests.

server {
    listen 8000;

    location / {
        slice 1m;

        proxy_cache cache;
        proxy_cache_key $uri$is_args$args$slice_range;
        proxy_set_header Range $slice_range;
        proxy_cache_valid 200 206 1h;
        proxy_pass http://127.0.0.1:9000;
    }
}

Do not assume that space character follows the method name, just pass it
explicitly.

The fuss around it has already proved to be unsafe, see bbdb172f0927 and
http://mailman.nginx.org/pipermail/nginx-ru/2013-January/049692.html for
details.

If an upstream with variables evaluated to address without a port,
then instead of a "no port in upstream" error an attempt was made
to connect() which failed with EADDRNOTAVAIL.

The directive toggles conversion of HEAD to GET for cacheable proxy requests.
When disabled, $request_method must be added to cache key for consistency.
By default, HEAD is converted to GET as before.

If sub_filter directive was only specified at http{} level, sub filter
internal data remained uninitialized.  That would lead to a crash in runtime.