> Suppose we agree that supporting stackable filesystems is a desirable
> goal. Consider stacking one filesystem -- a "wrapper" VFS, with
> as-yet-undetermined extra semantics -- over an underlying VFS which
> supports semantics like UFS holes, *and* associated growth
> (allocation) of holes, *and* correctly implementing consistency
> between, on the one hand, mmap()ed access and read()/write() access.
> 
> Once you start trying to make that work, I find that you very, *very*
> quickly break what should be clean abstraction boundaries between
> filesystem layers.  

is it related to my proposal?
can you give me examples?

> I'm not sure my email skills are up to illustrating with a specific
> case, right now; can I reply in more detail around noon tomorrow,
> US/Pacific time?

yes, please.

YAMAMOTO Takashi