Was support for VFS stacking removed? I was warned of such a year or
two ago, and stopped my VFS efforts at that point (also trying to
intercept file system calls).
Stacking filesystems in the BSD model doesn't work very well. It gets
expensive very quickly, and locking can become a nightmare. It's also
often not really what you want; the above is a good example of that.
The above being - ?
"also trying to intercept file system calls"
SunOS is a proof that BSD model and stackable VFS do work well
together.
Opinions on that differ to a considerable degree.
The reason locking in an operating system such as a BSD would
problematic is because VFS is very complex.
It needed flattening back in 1998 right after VM became stable. Now to
support MP kernel locking is more complex than ever. That does not
mean that things ought to be this way. Implementing a stackable FS is
relatively straightforward if you start from the right place - vnode
interface. If done correctly stackable does not create any problems
with locking that would not be present in a leaf design.
These are interesting assertions, but without any real application in
the current
context. I would caution anyone else reading this that your opinions
are certainly
not widely held.
In particular, fan-in, fan-out and partial unwinds are all necessary
for stacking
filesystems (if you don't want unwinds, you need reservations which are
arguably
worse). Locking these is painful at best, and nesting vnodes is never
not expensive.
= Mike
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Darwin-dev mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
http://lists.apple.com/mailman/options/darwin-dev/email@hidden
