Placing the filesystem into user libraries exo-kernel style is another
orthogonal issue. I don't want to start an exo-kernel implementation
right now, especially not without doing it completely and systematically
for all of the filesystem.
I'm not sure what you mean by "completely and systematically for all of
the filesystem"; I think a user library approach has the advantage that
it indeed works for all filesystems (not just one specialized filesystem
like NTFS 5), and for all operating systems (not just Linux). This is a
big win!
I also take issue with your calling what I described in my straw-man
proposal as an exo-kernel approach. There isn't the user->kernel->user
mode transition which is typical of standard exo-kernel approaches (and
which generally incur enough of a performance penalty to be
problematic).
What I was demonstrated was a pure user-mode-only approach, just to show
it was possible. In fact, if people want to play with filters that show
different views of the file, I would strongly encourage that it be first
prototyped using a userspace library approach, for the following
reasons:
1) Application writers will likely not use this thing unless there is a
way to get the same functionality across a wide number of Unix
platforms.
2) It is easier to test out approaches and semantics in user-mode than
in kernel mode. Doing it in kernel mode first risks doing a lot of work
which then either has to be thrown away, or which the kernel programmer
ends up clinging to because it represents so much investment, in spite
of the fact that application writers are telling him that this is not
what they want. Let's get the semantics and the functionality and the
interfaces right first, before we try thrusting it into the kernel.
3) The Unix way is to try doing things using a decomposed tool chain or
user space first, and only then pushing functionality into an integrated
program or into the kernel if absolutely necessary. If it turns out that
performance and maintainibility and functionality is perfectly adequate
using a pure user-mode approach, then perhaps, just maybe, it means it
doesn't belong in the kernel.
- Ted
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