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

[Lars Ellenberg]

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.

-- 
: Lars Ellenberg
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com

DRBD® and LINBIT® are registered trademarks of LINBIT, Austria.
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