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

Martin Gombač martin at isg.si
Fri Oct 30 23:11:06 CET 2009

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.


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])?


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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