Unioned filesystem mountpoints

Let's assume we have two servers set up as before:

Server A
A QNX 4 filesystem. Its mountpoint is /. It contains the files bin/true and bin/false.
Server B
A flash filesystem. Its mountpoint is /bin. It contains the files ls and echo.

Note that each server has a /bin directory, but with different contents.

Once both servers are mounted, you would see the following due to the unioning of the mountpoints:

Server A
Servers A and B
Server B
Server A
Server B
Server A

What's happening here is that the resolution for the path /bin takes place as before, but rather than limit the return to just one connection ID, all the servers are contacted and asked about their handling for the path:

DIR *dirp;
dirp = opendir("/bin", ...);

which results in:

  1. Server B receives a null path, since the request came in on the same path as the mountpoint.
  2. Server A receives the path "bin", since its mountpoint was "/".

The result now is that we have a collection of file descriptors to servers who handle the path /bin (in this case two servers); the actual directory name entries are read in turn when a readdir() is called. If any of the names in the directory are accessed with a regular open, then the normal resolution procedure takes place and only one server is accessed.