Re: Stacked file systems - why not and what instead?
Re: Stacked file systems - why not and what instead?
- Subject: Re: Stacked file systems - why not and what instead?
- From: Marek Kozubal <email@hidden>
- Date: Wed, 3 Mar 2004 01:03:06 -0500 (EST)
On Wed, 3 Mar 2004, Ragnar Sundblad wrote:
>
Background:
>
We are depending on the file system AFS in our environment
>
of different unix flavours. The AFS client is, as I
>
understand it, very much a stacked file system.
What you would do, is create a brand new file system, that's not layered,
and use UNIX sockets to do all file IO through a userland daemon, ala
webdav. This would remove any need for a layered file system.
>
My questions:
>
Could this be implemented at all without using a stacked
>
file system, and what would be the apple recommended way
>
of doing it? (Of course one could use a separate cache
See above...
>
I'd be happy if someone could explain both of these statements
>
from Q&A 1242:
>
"
>
... it's not possible to create a stacking VFS plug-in that:
>
1. works reliably,
Writing file systems is never a fun task, there are a lot of locking
issues involved and any minor mess up here and you'll be KP'ing some place
or another. They can also be *very* hard to track down.
>
2. does not severely impinge on system performance
In a layered file system, you consume resources more quickly. The kernel
keeps N vnodes alive at any given time, if you're doing a layered file
system then each vnode really now uses 2 vnodes, so you've effectively
changed N to N/2.
>
3. has any hope of binary (or even source-level) compatibility
>
with future systems.
Apple makes no promises not to completely change how all this stuff works
at some future date. The only Kernel interface that is not going to
change drastically I believe is the IOKit.
>
Would a stacking file system have (significantly) more
>
compatibility problems than a non stacking file system?
I've written a stacking file system in Mac OS X, we've not really seen any
more issues than I think I'd see w/a non-stacked file system, but I must
just have been lucky so far...
>
Wasn't there some ideas about a cleaner and more static
>
file system api for darwin?
It would be nice if Apple decided how they wanted it to work, and then
implelemnted it. Though I really doubt we'll see anything for 10.4.
---
Marek Kozubal
email@hidden
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