On Nov 3, 2005, at 11:50 AM, Steven M. Bellovin wrote:

> In message <20051103191902.GB547@hanele.ics.muni.cz>, Jachym  
> Holecek writes:
>>> Cool. We could further have /dev union mounted to avoid
>>> the mmap() dance + saving a few bytes of code. I'll test it
>>> this night.
>>
>> Patch below (modified version of Dave's one).
>>
>>> [Steven M. Bellovin]
>>> Last time I tried it, I couldn't union-mount a tmpfs over  
>>> anything.  It
>>> would be nice if that worked.
>>
>> Some naive tests seem to work for me (tmpfs union-over ffs2). Do you
>> see the problem in fresh -current as well?
>>
>
> Here's an example of a failure that doesn't occur with mfs:

Don't use a unionfs ... use a union mount!  That's different from  
unionfs.  unionfs is broken and buggy, and I would never trust it for  
a production system at this point.  It needs too much work.

Do a "mount -t tmpfs -o union swap /dev" instead.

>
> # mount -t tmpfs /dev/wd0b /mnt
> # mount
> /dev/wd0a on / type ffs (local)
> tmpfs on /tmp type tmpfs (local)
> kernfs on /kern type kernfs (local)
> procfs on /proc type procfs (local)
> tmpfs on /mnt type tmpfs (local)
> # mount -t union /mnt /usr/src
> mount_union: /mnt on /usr/src: Operation not supported
> # mount -t union /usr/src /mnt
> # mount
> /dev/wd0a on / type ffs (local)
> tmpfs on /tmp type tmpfs (local)
> kernfs on /kern type kernfs (local)
> procfs on /proc type procfs (local)
> tmpfs on /mnt type tmpfs (local)
> <above>:/usr/src on /mnt type union (local)
> # umount /usr/src
> umount: /usr/src: not currently mounted
> # umount /mnt
> # umount /mnt
>
>
>
> # mount -t mfs /dev/wd0b /mnt
> # mount
> /dev/wd0a on / type ffs (local)
> tmpfs on /tmp type tmpfs (local)
> kernfs on /kern type kernfs (local)
> procfs on /proc type procfs (local)
> mfs:1661 on /mnt type mfs (synchronous, local)
> # mount -t union /mnt /usr/src
> # umount /usr/src
> # mount -t union /usr/src /mnt
> # umount /mnt
> # umount /mnt
>
>
> 		--Steven M. Bellovin, http://www.cs.columbia.edu/~smb
>

-- thorpej