State of unionfs?

[K. Macy]

Everything I've been told is that unionfs has essentially never worked
right. FreeBSD's VFS semantics and vnode life cycle make it very
difficult to implement correctly.

-M

On Wed, May 18, 2016 at 4:26 PM, Johannes Totz <johannes at jo-t.de> wrote:
> On 18/05/2016 10:27, Patrick M. Hausen wrote:
>> Hi, all,
>>
>> we were looking for a way to get overlay/copy-on-write mounts for
>> ZFS datasets to ease jail management.
>>
>> Google turned up this old thread:
>> https://lists.freebsd.org/pipermail/freebsd-fs/2010-September/009221.html
>>
>> So, clearly in September 2010 mount_unionfs(8) was not supported
>> for ZFS datasets.
>>
>> A quick check on a current RELENG-10.3 system showed that this has
>> changed .Union-mounting one dataset on top of another does indeed
>> work at a superficial glance.
>>
>> Yet the manpage for mount_unionfs(8) still contains this disturbing
>> note:
>>
>> BUGS
>>      THIS FILE SYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK)
>>      AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM.  USE AT YOUR OWN
>>      RISK.  BEWARE OF DOG.  SLIPPERY WHEN WET.  BATTERIES NOT INCLUDED.
>>
>> Is this still the case? Are there alternatives to our approach.
>>
>> What we would like to implement is e.g. a standard pre-populated /etc for each
>> jail with only modified files being written to a separate per-jail dataset.
>> Much like NanoBSD does when populating the /etc mfs at boot.
>>
>> While we can create a clone from a central snapshot for each jail, the
>> problem with this way is that we cannot exchange the base snapshot later,
>> e.g. after a major system update for the jail in question. Which is precisely
>> the intention in the first place ;-)
>>
>> Thanks for any hints
>> Patrick
>>
>
> I've used unionfs with zfs for a while now. Seems ok.
> But beware of nesting any mounts into either lower or upper layer. Files
> created in there may not appear in the right place. They used to, but
> that broke at some point.
>
> I'm now moving away from unionfs, and doing a simple zfs clone. When
> it's time to upgrade, copy data files separately. Config files are
> tracked with Mercurial.
>
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