ideas about a unioning file system

Andrew Reilly areilly at
Mon Jun 23 23:52:12 PDT 2003

On Tue, 2003-06-24 at 16:29, Terry Lambert wrote:
> Andrew Reilly wrote:
> > 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/vinum/vinum0       /usr            ufs     rw,union        0
> > 2
> Actually, this is the union mount option, which isn't the same
> thing as unionfs.

Aah.  OK.  I wondered a bit about that myself.

So: what's unionfs?  Does the union option in fstab invoke mount_union,
or is union mounting something that mount manages for itself,
independent of the FStype?

Having looked at mount_union, I don't think that I like the
copy-to-top-layer on write semantics for objects that exist in the
bottom layer.  I hope that's not happening to my system.

> It's a mount option, not a file system.  8-).

How could it be otherwise?

Andrew Reilly <areilly at>

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