geom_mirror silently upgrading metadata [Was: cvs commit: src UPDATING]

[Ollivier Robert]

According to Maxim Sobolev:
> Not really relevant to the change in question, but I think that the whole 
> idea of geom_mirror updating on-disk metadata automagically is not  very 
> well thought out. For example one could try booting 7.x kernel on 6.x 
> system just to see how well it goes with the intention to revert back if it 
> doesn't work out well. We have excellent tool called nextboot (8) that 
> really helps doing it safely or semi-safely even remotely over ssh. In the 
> worst case you just need to cycle the power to return to the previous 
> configuration.
> 
> Automatic conversion makes it impossible to go back without some heavy 
> manual intervention at console necessary to boot off the disk directly and 
> re-creating/re-syncing the mirror after that. I've run into exactly this 
> issue today, with the target machine stuck in unbootable state on another 
> continent many thousand miles away.

I completely agree on that one, I got bitten by this during my upgrade from
6.1-STABLE to 7.0-RELEASE on a gmirror-ed machine.  That is a complete
violation from POLA and harmful to say the least.
-- 
Ollivier ROBERT -=- FreeBSD: The Power to Serve! -=- roberto at keltia.freenix.fr
Darwin sidhe.keltia.net Version 9.2.0: Tue Feb  5 16:13:22 PST 2008; i386