ideas about a unioning file system

Andrew Reilly areilly at
Mon Jun 23 20:04:40 PDT 2003


On Fri, 2003-06-20 at 08:15, Mohammad Nayyer Zubair wrote:
> Has anyone extensively used freebsd unionfs? From a system/network
> administrator or from a kernel developer standpoint, what do you like
> about it and what you dont like about it?

I'm using unionfs thusly:

# Device                Mountpoint      FStype  Options         Dump   
/dev/ad0s1a             /               ufs     rw              0      
/dev/vinum/mirror       /home           ufs     rw              0      
/dev/vinum/vinum0       /usr            ufs     rw,union        0      

(sorry about the wrappage, cut and pasted from /etc/fstab.)

There are other lines in /etc/fstab, of course, but these are the bulk
of my workstation's file hierarchy.

After years of tiny root partitions that eventually caused grief and
breakage because of gradually expanding kernels/modules/cruft-in-etc,
this time around I've made root about 500M and put the whole FreeBSD
base system in there.  So a buildworld/kernel/installworld touches
everything in ad0s1a, and nothing else does (except /etc).

This leaves the problem of what to do with /usr/ports, /usr/X11R6,
/usr/local, /compat at ->usr/compat, /tmp at ->var/tmp, /var at ->usr/var

I could have mounted /dev/vinum/vinum0 somewhere like /usr1, and filled
/usr with a bunch of symlinks, and I did that for a while.  It's messy
though, and quite a few things, like ports, bother to find their "true"
path, so the /usr1 name leaks into config files and what-not.  Ugly.

Union mounting seems to be working for me.  I do my backups, and it's
not a super-heavily used system...  Touch wood.

> How should a unioning filesystem should behave? What specific features
> would you like it to have?

It should behave just the way it does: the stuff in /usr that was there
before the mount stays there, and gets modified and all, just as it
should be.  Anything that wasn't in /usr before the mount gets written
to the union partition.  Reads see both.

> Out of the previous efforts at a unioning file system like the Sun's TFS,
> 3DFS, Plan 9 and FreeBSD unionfs itself, which fs do you think came close
> to an ideal unioning file system?

What's wrong with the one that we have?


Andrew Reilly <areilly at>

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