ideas about a unioning file system

Poul-Henning Kamp phk at
Fri Jun 20 01:55:53 PDT 2003

In message <3EF2CAF1.4080605 at>, Donn Miller writes:
>David Schultz wrote:
>> Unionfs in FreeBSD has a number of implementation issues.  Many of
>> these stem out of locking problems and the fact that VFS layering
>> doesn't really work right in FreeBSD.  These are fixable details,
>> and perhaps FiST addresses some of these.
>That's one thing I was curious about.  Is there a need to redo the VFS 
>layer in FreeBSD?  Someone mentioned it as a possible to-do item.  If 
>so, would it make it easier to port new filesystems to FreeBSD?

There are certainly deficiencies in our VFS layer, but nothing compared
to our other challenges, so unless some external factor makes an
up/down/side-grade a good idea, I think we should focus on making it
work as it is.

The majority of the problems we have in this area can be nailed down
to only two issues:
	1. The very tricky semantics of rename(2)
	2. read/write/mmap coherency for stacked filesystems.

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

More information about the freebsd-fs mailing list