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

[mark at legios.org]

>
>>IMHO metadata update should be performed if and only if explicitly
>>requested by the administrator.
>
> Agreed.  It's especially worrying that there's absolutely no warning
> that a particular version of geom_mirror has a different metadata
> format and loading it will make your gmirror unusable with an older
> gmirror.  IMHO, any geom changes changes that prevent reversion
> should be noted in UPDATING (at the very least).
>
Seconded. ZFS appears to have functionality similar to this whereby you
can perform an upgrade using zpool upgrade to update the metadata.
(Also, on loading zfs.ko it notifies you of the ZFS version it is/supports)