Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. The client’s view
Both fs-nfs (the network filesystem) and your caching filesystem (fs-cache)have
registered themselves for the same prefix, namely “/nfs.” As we mentioned above,
this is fine, normal, and legal under Neutrino.
Let’s say that the system just started up and your caching filesystem doesn’t have
anything in it yet. A client program tries to open a file, let’s say
/nfs/home/rk/abc.txt. Your caching filesystem is “in front of” the network
filesystem (I’ll show you how to do that later, when we discuss resource manager
implementation).
At this point, the client’s open() code does the usual steps:
1 Message to the process manager: “Who should I talk to about the filename
/nfs/home/rk/abc.txt?”
2 Response from the process manager: “Talk to fs-cache first, and then
fs-nfs.”
Notice here that the process manager returned two sets of ND/PID/CHID/handle; one
for fs-cache and one for fs-nfs. This is critical.
Now, the client’s open() continues:
1 Message to fs-cache: “I’d like to open the file /nfs/home/rk/abc.txt for
read, please.”
2 Response from fs-cache: “Sorry, I’ve never heard of this file.”
At this point, the client’s open() function is out of luck as far as the fs-cache
resource manager is concerned. The file doesn’t exist! However, the open() function
knows that it got a list of two ND/PID/CHID/handle tuples, so it tries the second one
next:
1 Message to fs-nfs: “I’d like to open the file /nfs/home/rk/abc.txt for
read, please.”
2 Response from fs-nfs: “Sure, no problem!”
Now that the open() function has an EOK (the “no problem”), it returns the file
descriptor. The client then performs all further interactions with the fs-nfs resource
manager.
The only time that we “resolve” to a resource manager is during the open() call. This
means that once we’ve successfully opened a particular resource manager, we will
continue to use that resource manager for all file descriptor calls.
So how does our fs-cache caching filesystem come into play? Well, eventually, let’s
say that the user is done reading the file (they’ve loaded it into a text editor). Now they
want to write it out. The same set of steps happen, with an interesting twist:
1 Message to the process manager: “Who should I talk to about the filename
/nfs/home/rk/abc.txt?”
April 30, 2009 Chapter 5 • Resource Managers 197