Strange md/unionfs issue

[David Ehrmann]

Gavin Atkinson wrote:
>
> On Sat, 10 Oct 2009, David Ehrmann wrote:
>
>> I have a system running on a flash disk.  To prevent it from wearing 
>> out quickly, I don't run with a swap and mount /tmp and /var from 
>> memory. Because /var needs some directories and contains files I 
>> probably should save from time to time, I mount it from flash, mount 
>> /var to /var_real so I can access the underlying files, and then I 
>> mount an md device on top of /var as a unionfs.  Initially, it seems 
>> to work, but seconds later, the unionfs seems to fail.
>
> When the mount "fails", is it still shown in the output of mount(1)?
The output of mount is the same; it reports that a md-backed unionfs is 
on top of /var.
>
> It appears that this problem is the same as is reported in PR 
> kern/131441, which has so far not had any activity.  It appears that 
> this bug has existed since at least 7.1-RELEASE.
It does seem to be the same bug (or similar).   With mine, I mounted a 
unionfs on top of a mount point that a nullfs was based on, not the 
nullfs's mount point.  I think the bug in kern/131441 was with a unionfs 
over a nullfs.  This probably means md's off the hook as the culprit, 
and the same thing would happen with ufs on /dev/foo.