Re: [9fans] Re: 9P, procedure calls, kernel, confusion. :(

[Anant Narayanan <anant@kix.in>]

Hi,

> One more point, I googled a lot on "kernel resident file systems and
> non kernel resident file systems", but I could not find a single
> useful link. It would be great if you could specify the difference
> between the two. I wish that eases up the situation a bit.

I don't claim to know much about how 9P or Plan 9 work, but I will
attempt to answer.

9P is very much like Linux's VFS. It defines how user-space
applications can access files; whether they are stored locally, or on
the network, or whether the information is generated on-the-fly by the
kernel from its internal data-structures is of no consequence - 9P
abstracts all that.

Accessing files from within the kernel is a different ball-game
(that's true for every kernel). Since you don't have the 9P style
access anymore - The 'Dev' structure essentially replaces it. These
structures point to methods are quite analogous to the 9P operations.
The methods do the work of implementing the file operations - and
these would all be different for files depending on whether they are
stored on local disk, produced synthetically or accessed over a
network. This makes it easier to work with files in the kernel since
all the operations are delegated further to the 'Dev' structures, each
one doing what it knows best; quite similar to what happens with 9P in
user-space.

Charles mentions one such 'Dev', the mount driver, which merely passes
on the received request as 9P messages to the file descriptor
(possibly connected to a remote 9P server).

"Kernel resident filesystem" in this context simply means a filesystem
which was created for use by the kernel; this may or may not be
visible to user-space applications - I'm not too sure. To sum up, you
use the 9 primitive operations provided by each 'Dev' when you work
with kernel-resident filesystems, while all other filesystems are
dealt with using regular 9P.

I hope I'm right, and that this helps.

--
Anant