[DRBD-user] DRBD 9: 3-node mirror error (Low.dev. smaller than requested DRBD-dev. size.)

[Robert Altnoeder]

On 7/19/19 9:33 PM, Paul Clements wrote:
> Any ideas? Something I'm doing wrong? I didn't specify a size
> anywhere, so not sure why this isn't working.

Apparently, you used the default values when you initially created the
DRBD meta data for this installation. Since you had only two nodes
listed in the configuration file and did not specify otherwise, drbdadm
created meta data for a two node cluster.
Then you added another node to the configuration file, and now you have
three nodes in the configuration file, so drbdadm creates meta data for
a three node cluster. This will create meta data with 2 peer slots
instead of only 1 peer slot, which requires approximately twice the
amount of space. Since you are using internal meta data, the net size of
the DRBD device will be smaller.

You have two problems here:
1. The net size of the storage on the third node is smaller than on the
other two nodes, so DRBD can not resync to that node
2. The other two nodes have only one peer slot each, so they cannot
connect to a third node

You need to recreate the meta data with at least 2 peer slots to run a
three node cluster.

To make space for the additional peer slots, you will have to either
- increase the size of the storage device
or
- decrease the size of the data (e.g. filesystem) on the storage device
or
- switch to external meta data

br,
Robert