"Reverse union" mount possible?

Tim Robbins tjr at freebsd.org
Thu Nov 20 16:53:25 PST 2003

On Thu, Nov 20, 2003 at 02:41:43PM +0300, Alex Povolotsky wrote:

> Is it somehow possible, or how complex patch will require to get "reverse union"
> mount, with first lookup in underlying system, and file creation there as well?

I believe unionfs can do this (mount_unionfs -b option). mount's "union"
option only has a small subset of unionfs's features.

> Right now, I'm readonly mount_null'ing /bin, /sbin, /usr/bin, /usr/sbin,
> /usr/lib, /usr/include, /usr/libexec, /usr/share. With a dozen jails, there
> are too many mounts to my liking, and about twice a week I experience panic.
> Probabily it's nullfs-related.

Nullfs is known to be buggy in -stable. In particular, it seems to deadlock
under load / when vnodes start getting recycled.


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