zfs: allow to mount root from a pool not in zpool.cache

[Justin T. Gibbs]

On Oct 3, 2012, at 2:53 AM, Pawel Jakub Dawidek <pjd at freebsd.org> wrote:

> On Sat, Sep 22, 2012 at 10:59:56PM -0600, Justin T. Gibbs wrote:
>> On Sep 22, 2012, at 10:28 AM, Andriy Gapon <avg at freebsd.org> wrote:
>> 
>>> 
>>> Currently FreeBSD ZFS kernel code doesn't allow to mount root filesystem on a
>>> pool that is not listed in zpool.cache as only pools from the cache are known to
>>> ZFS at that time.
>> 
>> I've for some time been of the opinion that FreeBSD should only use
>> the cache file for ZFS pools created from non-GEOM objects (i.e.
>> files).  GEOM tasting should be used to make the kernel aware of
>> all pools whether they be imported on the system, partial, or
>> foreign.  Even for pools created by files, the user land utilities
>> should do nothing more than ask the kernel to "taste them".  This
>> would remove code duplicated in user land for this task (code that
>> must be re-executed in kernel space for validation reasons anyway)
>> and also help solve problems we've encountered at Spectra with races
>> in fault event processing, spare management, and device arrival and
>> departures.
>> 
>> So I'm excited by your work in this area and would encourage you
>> to "think larger" than just trying to integrate root pool discovery
>> with GEOM.  Spectra may even be able to help in this work sometime
>> in the near future.
> 
> GEOM tasting would most likely require rewriting the code heavly.
> Also note that you can have pools in you system that do match your
> hostid, but user decided to keep exported and such pool should not be
> configured automatically. Not a huge problem probably as there is pool
> status somewhere in the metadata that we can use to see if the pool is
> exported or not.

This topic came up during ZFS day last week.  It turns out that the OS-X
port of ZFS already does this and Don Brady said he's happy to share
patches.

I don't see any reason why this type of solution cannot be upstreamed
as well.  On Illumos, user land can maintain the cache file and use it
to ask the in-kernel ZFS code to "taste" devices, minimizing the amount
of divergence.

--
Justin