[DRBD-user] Split brain in a dual primary configuration

[Martin Gombac(]

Well,

it's actually being done in such a way. Apparently it even has it's own 
manual chapter. :-)
http://www.drbd.org/users-guide/ch-xen.html
And a blog entry:
http://blogs.linbit.com/florian/2007/09/03/drbd-806-brings-full-live-migration-for-xen-on-drbd/

I guess that's the way to go. :-)

Regards,
M.

Martin Gombac( wrote:
> Hi,
> 
> mind that i'm no expert and can be completely wrong but..
> 
> LVM works on top of drbd in active/passive mode only.
> For active/active you need CLVM (and all of that RH Cluster Suite S**t)
> 
> It's the same as with filesystems, if you want to have it mounted on
> many locations at the same time, you need locking, so that no two nodes
> write at the same spot/block at the same time. LVM by itself doesn't
> guarantee that.
> 
> But to inform you, you're not only one who tried that setup. :-) I'm
> currently looking for appropriate solution.
> 
> One might be:
> Disk <-> LVM <-> DRBD[X] <-> domU[X].
> Where each DRBD instance is one virtual machine.
> It would work, only if during live virtual machine migration from host A
> to host B, writing on host B starts _after_ all writing on host A ceases.
> 
> Does anyone know if this would work and if XEN can/will write
> concurrently during live migration on both backing devices (DRBD[X])?
> 
> Regards,
> M.
> 
> 
> Jean-Francois Chevrette wrote:
>> Hello,
>>
>> On 09-10-30 3:07 PM, Gianluca Cecchi wrote:
>>> Hello,
>>> the message is self explanatory: in drbd.conf you define the policy to
>>> "disconnect" when you get a split brain (sb) deriving from a 2-primary
>>> scenario.
>>> And so does drbd...
>>
>> But what else would be more appropriate for such a situation? In fact, 
>> that's what we want to do, have both nodes to disconnect. We don't 
>> want either of them to become secondary.
>>
>> Is it acceptable to have both nodes remain primaries while they are 
>> disconnected and expect them to sync to each other properly when they 
>> are connected again?
>>
>>> btw: having you dual primary and LVM, are you using also CLVMD?
>>> Otherwise if you do modifications on one VG (such as add an lv) you
>>> don't see them immediately, because you don't have cluster locking...
>>
>> We are not using clvm. When a new VG or LV is created, we see it 
>> immediately on the second node. Maybe Citrix XenServer has a mechanism 
>> so that LVM is reloaded on both nodes when a new VM is created on the 
>> cluster?
>>
>>
>> Regards,
> 
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