External data buckets
The File bucket, as we have already seen, enables us to insert a file
(or part) file into the data stream. Although we had to stat it to find
its length, we didn't have to read it. If sendfile is enabled, the
operating system (through the APR) can optimise sending the file.
The mmap bucket type is similar, and is appropriate to mmaped files.
APR may convert file buckets to mmap internally if we (or a later filter)
read the data.
Two other more unusual bucket types are the pipe and the socket, which
enable us to insert data from an external source via IPC.
