Does named pipe modify the filesystem? If I create a named pipe and then read/write on it, is the filesystem where the named pipe resides affected? I.e. is the data buffered on the filesystem until read, or does it reside in memory only?

The file object itself is created in the filesystem, but no data is stored in a file system. From the mkpipe(3) manpage:


A FIFO special file is similar to a pipe, except that it is created in
a different way. Instead of being an anonymous communications channel,
a FIFO special file is entered into the file system by calling
mkfifo().


About the only time the data might be stored on disk is during hibernation when memory is written to the swap space, including buffers - however this an corner case.