MFC of ZFSv15

Matthew Seaman m.seaman at
Sun Sep 19 17:41:41 UTC 2010

On 19/09/2010 17:36:01, Dan Mack wrote:
> But I should be able to boot my ZFSv14 root pool using the ZFSv15
> build of FreeBSD, correct?   But the problem scenario would be when
> I've upgraded my root pool to v15 and I attempt to boot it with v14
> boot loader.  At least that is what I think ...

Yes.  The bootloader is not prescient, so  bootloader compiled against
v14 can't cope with a zpool using v15.  It's only the on-disk format
that counts in this: zfs software will operate perfectly well with older
on-disk data formats.

> I guess what I'm getting at is ... you should be able to buildworld,
> installkernel, reboot, installworld, reboot without worry.   But
> after your run 'zpool upgrade', you will need to re-write the
> bootcode using gpart on each of your root pool ZFS disks.

If you want to be completely paranoid, you could update the bootcode on
your boot drive (or one out of a mirror pair, if that's what you're
using) at the point of running installkernel and way before you run
'zpool upgrade'.  In theory, should this go horribly wrong and you end
up with an unbootable system, you can recover by booting the 8.0 install
media into FIXIT mode and reinstalling the bootblocks from there (or
booting from the other disk in your mirror set).  Once you've got a
system you know will reboot with the new bootblocks, then go ahead and
with installworld and updating the zpool version.

> Am I understanding this correctly ?

Yep.  That's quite right.  Running 'zpool upgrade -a' is one of those
operations you can't easily reverse, so designing an upgrade plan where
you can stop and back-out at any point is quite tricky.  Fortunately,
the risk of things going wrong at the point of running zpool upgrade is
really very small, so for most purposes, just ploughing ahead and
accepting the really very small risk is going to be acceptable.



Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
                                                  Flat 3
PGP:     Ramsgate
JID: matthew at               Kent, CT11 9PW

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