[DRBD-user] [Drbd-dev] 3-node active/active/active config?

[Jiann-Ming Su]

On Tue, Nov 17, 2009 at 4:42 AM, Lars Ellenberg
<lars.ellenberg at linbit.com> wrote:
> On Sun, Nov 15, 2009 at 01:08:30AM -0500, Jiann-Ming Su wrote:
>> Don't know if this has been discussed before, but it seems like with
>> 8.3's dual primary support, it's possible to set up a 3-node, all
>> active drbd cluster.
>>
>> Each node would be dual primary with each other node.  Node A would
>> form drbd0 with node B and drbd1 with node C.  Node B would have drbd0
>> with A and drbd2 with C.  And, C would have drbd1 with A and drbd2
>> with B.
>>
>> Then you can use software raid to create md0 on each node with its
>> respective drbd devices.
>>
>> Noce A:  md0 (drbd0, drbd1)
>> Node B:  md0 (drbd0, drbd2)
>> Node C:  md0 (drbd1, drbd2)
>>
>> Then format md0 on each node with a cluster filesystem like ocfs2 with
>> each node as a member of the cluster.
>>
>> Do the drbd experts here have any thoughts on how this idea would go
>> horribly wrong?  Thanks for any input and feedback.
>
>
>  A-MD(A-DRBD0(A-sdx), A-DRBD1(A-sdy))
>  B-MD(B-DRBD0(B-sdx),                 B-DRBD2(B-sdy))
>  C-MD(                C-DRBD1(C-sdx), C-DRBD2(C-sdy))
>
>
> So you write block N on A-MD.
> It will hit A-DRBD0, and A-DRBD1,
>  which will hit local disks A-sdx, A-sdy,
>  and via DRBD replication B-sdx, and C-sdx.
>
> Still with me?
> Great.
>
> Now, you want to read that block N on B,
> which decides in its read balancing path to fetch
> block N from B-sdy.
>
> It will surely get _some_ data from there,
> but certainly not the data you just wrote on A-MD.
>
> To summ it up in one word:
> DON'T.
>

Thanks for the insight!  So there's no easy way to verify a write has
sync'd across all the nodes?  For the application I want to use this
type of  3-node config on, I think I'm willing to sacrifice the
performance for the data replication.

-- 
Jiann-Ming Su
"I have to decide between two equally frightening options.
 If I wanted to do that, I'd vote." --Duckman
"The system's broke, Hank.  The election baby has peed in
the bath water.  You got to throw 'em both out."  --Dale Gribble
"Those who vote decide nothing.
Those who count the votes decide everything.”  --Joseph Stalin