what is gab seeding ?

Seeding is to protect against pre-existing split-brain, when a node comes up, as oppose to I/O fencing which is to protect against split-brain that happens when cluster is running.

So seeding is only applicable when a node starts and it is to stop 2 or more sub-clusters starting which can't "see" each other.  So when a node starts if it can see a seeded cluster, then it seeds and joins the cluster and if it can't see any nodes then it waits.  Unseeded nodes wait together until the number of nodes defined by the

gabconfig -c -n number_of_nodes_required_to_seed

line in /etc/gabtab.  

So suppose you have 3 nodes and you reboot all 3 together and one does not start and you have "gabconfig -c -n 3" in /etc/gabtab, then the 2 nodes that come up will wait together for the 3rd node because it could be the case that the 3rd node is a seeded cluster that they cannot see and is running the application and therefore if the 2 nodes were to form a cluster and start the same application, you would get split-brain.

If you know 3rd node is down, then you need to manually seed (regardless of whether llt is working on all nodes), and then you run "gabconfig -cx" on one of the two nodes and this seeds the cluster.  Suppose in, this scenario, you later reboot one of the 2 nodes in the seeded cluster and the 3rd node is still down, then when the rebooted node comes up, in joins the cluster because the remaining 1 node it can see is a seeded cluster, so it doesn't matter there are only 2 nodes in the cluster.

You don't have to define the total number of nodes for "-n" flag in /etc/gabtab, but this is the safest option, but you should specify at least more than half, so example, "-n 2" for a 3-node cluster and -n 3" for a 4 node cluster)

Mike