Gmirror - how to do?

[Karl Denninger]

On Sat, Feb 05, 2005 at 09:12:37PM +0100, Pawel Jakub Dawidek wrote:
> On Sat, Feb 05, 2005 at 01:57:05PM -0600, Karl Denninger wrote:
> +> Howdy;
> +> 
> +> Another quickie someone may know how to handle.
> +> 
> +> I've got a 2-drive RAID1 mirror I wish to back up.
> +> 
> +> The easy way appears to be to attach a third drive, let it sync, detach it
> +> and then you have a backup, right?
> +> 
> +> So I do the following:
> +> 
> +> 	atacontrol attach 2 (attach new disk on external adapter)
> +> 	gmirror insert boot ad4s1 (insert the backup into the existing mirror)
> +> 	wait (while the disk synchronizes - 3-4 hours)
> +> 	gmirror remove boot ad4s1 (remove third copy from mirror)
> +> 	atacontrol detach 2 (remove device from the system)
> +> 
> +> Now I can go pull the carrier "cleanly".
> +> 
> +> Except for one small problem - when you do this, then try to boot the
> +> backup volume it fails, because gmirror has marked the metadata as "do 
> +> not use" when you removed it, yet the /etc/fstab entries all point to a
> +> mirror that isn't there.
> +> 
> +> So... how do you accomplish this?
> +> 
> +> Detach the BUS underlying the drive without warning gmirror first (e.g.
> +> "atacontrol detach 2", without the preceding "gmirror remove"), thereby 
> +> forcing a "dirty" disconnect?  I'd rather not, although if I must, that I 
> +> suppose would work.  However, if I do this, then gmirror thinks I have a
> +> third volume present, and as a consequence as soon I re-init that channel
> +> and geom sees the disk it will immediately begin a rebuild (whether this is
> +> bad or not I suppose is a matter of interpretation)
> 
> In you case I suggest do this:
> 
> 1. atacontrol attach 2
> 2. gmirror insert boot ad4s1
> <wait for sync>
> 3. gmirror remove boot ad4s1
> 4. gmirror label boot ad4s1
> 5. atacontrol detach 2
> 
> In 4th step, you labeling only one provider with the same name.
> It will be tasted, but ignored, because mirror 'boot' is already
> configured:
> 
> GEOM_MIRROR: Device boot already configured.
> 
> Now it should be bootable.
> 
> Remember not to boot the main machine with this disk inside, as it can
> be tasted before your main 'boot' mirror. Inserting this disk after
> boot, when your 'boot' mirror is configured should be safe.

Nope, won't work.

The mirrors potentially have different PHYSICAL slice sizes (remember
this debate a while back on this list?) and if I do this, I'm guaranteed to
screw the partition table, as the fdisk size of the slice table will be
picked up.

--
-- 
Karl Denninger (karl at denninger.net) Internet Consultant & Kids Rights Activist
http://www.denninger.net	My home on the net - links to everything I do!
http://scubaforum.org		Your UNCENSORED place to talk about DIVING!
http://www.spamcuda.net		SPAM FREE mailboxes - FREE FOR A LIMITED TIME!
http://genesis3.blogspot.com	Musings Of A Sentient Mind