gmirror (was Re: It's time to bite the bullet and do a major upgrade...)

[Jonathan McKeown]

On Wednesday 15 November 2006 16:58, John Nielsen wrote:

> It is possible to convert "regular" devices into gmirror members after they
> have data on them, but unless you're extremely careful there's a small risk
> of the gmirror metadata sector overlapping a data sector.

OK, I see the warning in the gmirror(8) manpage that gmirror metadata 
overwrites the last sector of the provider.  Is that sector more likely, or 
less likely, to be in use than any other sector on a non-full disk? If it's 
equally or less likely the risk is extremely small - which I know is no 
consolation when it happens!

In this case, I'm doing something of a ``stunt upgrade'' anyway: I have two 
remote boxes to upgrade to 6.1, one of which is running 5.4-RELEASE and one 
4.8-RELEASE. Both boxes have 80GB drives, and on my last flying visit I added 
to each box a blank 80GB drive and a null-modem serial link to a neighbouring 
ssh-accessible box.

The plan is to ssh to the neighbour box, establish a serial console on the 
upgrade target, install 6.1 from scratch over the network on the blank drive 
and then make it the only drive in a gmirror. Once that's done, data can be 
migrated from the original drive, which can then be added to the mirror.

I have successfully carried out the procedure on a box in my office (so that I 
could intervene when it all went horribly wrong, several times) and am in the 
process of documenting it: as I said earlier, I couldn't find an easy guide 
to all this anywhere - perhaps not surprising as it's an odd thing to want to 
do.

Jonathan