[DRBD-user] Feature suggestion for primary/primary configurations

Florian Haas florian.haas at linbit.com
Mon Nov 30 09:02:50 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.

On 2009-11-27 18:37, . Honmans wrote:
> If it is not possible now, would be possible in future for a
> primary/primary split-brain to be resolved by keeping track of "recently
> updated" data on the two nodes and merge the changes rather than rolling
> them back on one of the nodes as in
> http://www.drbd.org/users-guide-emb/s-resolve-split-brain.html ?

Nope. I mean it technically would be possible, but it would be a sure
fire way to _completely_ wreck your filesystem, which would be
blissfully unaware of such a merge. And then what would you do with a
block which was modified on both nodes? Sorry, forget that.

> Obviously a primary/primary configuration will be anarchy unless there
> is coordination between peers at a higher software level - such as the
> Proxmox VE software which "knows" which of the peers each VM is running on.
> Can DRBD make use of that information to synchronise regions of a
> resource in different directions - or would it be possible to add that
> as a feature at some future date?
> For example, if we have LVM on top of DRBD (say /dev/drbd0), and each VM
> has a distinct Logical Volume for its virtual disk...
> VM vm1 on LV vm1data - currently running on peer A
> VM vm2 on LV vm2data - currently running on peer B
> When resolving split-brain, the region of /dev/drbd0 occupied by LV
> vm1data is synced from peer A to  B, and the region occupied by vm2data
> is synced from B to A.
> In effect, there would be synchronisation of regions of resources rather
> that whole DRBD resources.

Use 2 DRBD resources.

> In the meaintime, any more information on resolving primary/primary
> split-brain conditions would be very welcome.

Don't let that happen. :)

When you run into replication breakage, you really need to kick one node
out of your cluster. Quickly. Check the drbd.conf manpage for the
"fencing resource-and-stonith" option, and boot one of your nodes.

There will be some more information about this in the DRBD User's Guide
with the next maintenance release.


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