freebsd stable - Jun 2008 - GEOM gotcha upgrading from 6.x to 7.x

Yesterday, I planned to try an upgrade from 6.x to 7.x on one of the
servers at work, on the assumption that I could reasonably easily
revert to 6.x if things went bad.  My confidence in being able to
do that was severely shaken when I saw "Upgrading metadata" messages
for both geom_mirror disks when the 7.x kernel booted.

Whilst I did not need to revert, some examination of the geom_mirror
source shows that (as I feared), a 6.x kernel will choke on a 7.x
geom_mirror.

I've looked through the various geom man pages, UPDATING, the relevant
commit message and various mailing lists and am unable to find any
mention of this.  Having the kernel automatically alter persistent
state on the host in a way that is incompatible with earlier kernels
is bad enough.  Doing so without any mechanism to revert the change
and with no "heads-up" warning of the change is, IMO, unacceptable.

I accept that it's sometimes necessary to make changes that are not
backward compatible or that are difficult to revert, such changes
are normally discussed in advance and come with heads-up warnings.
I can't think of any previous case where simply test booting a kernel
is enough to render your system unusable with an older kernel.

-- 
Peter Jeremy
Please excuse any delays as the result of my ISP's inability to implement
an MTA that is either RFC2821-compliant or matches their claimed behaviour.
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Hello,

On Sat, Jun 28, 2008 at 07:49:12AM +1000, Peter Jeremy
wrote:
> Yesterday, I planned to try an upgrade from 6.x to 7.x on one of the
> servers at work, on the assumption that I could reasonably easily
> revert to 6.x if things went bad.  My confidence in being able to
> do that was severely shaken when I saw "Upgrading metadata"
messages
> for both geom_mirror disks when the 7.x kernel booted.
> 
> Whilst I did not need to revert, some examination of the geom_mirror
> source shows that (as I feared), a 6.x kernel will choke on a 7.x
> geom_mirror.
I didn't try now, but from my experience it should be fairly easy to 
revert: Boot without gmirror from one the disks with the old kernel. 
Have an old world available (CD, not yet installworld'ed) an write 
gmirror label from single user mode with old metadata, add the other 
disk and rebuild (more or less just go the way you go to setup a 
bootable gmirror from an existing system).

You would be in more trouble if you had some kind of real RAID format 
(like a RAID 5 or something) on the disk, but gmirror is basically a set 
of metadata whilst any single disk of the mirror is actually in a 
format, that can be used without gmirror.

I agree, that this is not actually straigt forward (as downgrading isn't 
anyhow), but for the case "I see my 7.x kernel choking when upgrading 
from 6.x to 7.x" it should still be easy enough for an experienced 
admin.


Greetings,
		Oliver


-- 
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