[DRBD-user] Re: xen on drbd (or vice versa) [was: Changes in 0.7.12]

[Michael Paesold]

Lars Ellenberg wrote:

>> Another option would be the current trunk version of drbd
>> (Primary/Primary), but I guess you could not recommend using it. Is
>> the on-disk storage compatible with 0.7? Can 0.7 partitions be
>> upgraded to trunk?
>
> even then, it would be nonsense.

I don't think so...

> the client (guest) system will assume exclusive access, so no other
> system may have write access, even less so on the node that is
> drbd secondary.

The guest on the new host (after migration) will not start to run before the 
one on the old host will be destroyed. The problem is that Xen wants to 
setup the virtual device _before_ the old guest died. It will not be used at 
all until that point.

> you are on the wrong track.
>
> you have to switch the drbd device into primary mode before you start
> your guest systems. regardless of drbd version.

Of course I have to. But if I could switch it into primary before the other 
one was switched to secondary, this would work. As I read from the xen 
source code, there will be no filesystem access from both hosts at any time. 
(You could say Xen does the fencing.) Xen just wants to open the (raw) 
device too early.

Best Regards,
Michael Paesold