geom_mirror silently upgrading metadata

[Pawel Jakub Dawidek]

On Tue, Jul 22, 2008 at 02:44:42PM +0200, Dag-Erling Sm??rgrav wrote:
> Pawel Jakub Dawidek <pjd at FreeBSD.org> writes:
> > Just to be clear. I fully agree with you guys. What I could do about
> > that when I was working on gmirror (starting from the simplest
> > solution):
> >
> > 1. Skip disks which have version lower then what we have in the kernel.
> >
> > 2. Upgrade the on-disk metadata automatically.
> >
> > 3. Make gmirror kernel module to work with all the previous versions and
> >    add 'gmirror upgrade' command, so one can upgrade on-disk metadata.
> 
> 4. Allow an older mirror to be accessed r/o by a newer kernel, side-
>    stepping the issue of converting metadata back to the old format.
>    Require an explicit 'gmirror upgrade' to upgrade the metadata and
>    allow r/w access.
> 
> IIRC, this is what ZFS does.  I believe it would be a good compromise
> between 2 and 3.

ZFS does exactly 3. What you are suggesting could be done by not loading
geom_mirror.ko module and mounting file system read-only from one of the
mirror components. Of course automatic gmirror metadata upgrade is in
most of the cases a surprise. Although, one can still just boot it on an
older system and create mirror once again - 'gmirror label' command
won't touch the data.

-- 
Pawel Jakub Dawidek                       http://www.wheel.pl
pjd at FreeBSD.org                           http://www.FreeBSD.org
FreeBSD committer                         Am I Evil? Yes, I Am!
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