+ // The stream data itself, stored in a circular buffer.q
+ //
+ // We store our data in a file, so that we can send the data to the
+ // kernel only once (with write()). We then use sendfile() for each
+ // client, which effectively zero-copies it out of the kernel's buffer
+ // cache. This is significantly more efficient than doing write() from
+ // a userspace memory buffer, since the latter makes the kernel copy
+ // the same data from userspace many times.
+ int data_fd;