Hi,
I'm just going through the concepts of quorum and split-brains with a cluster in general, and trying to understand GlusterFS quorums again which I previously found difficult to accurately understand.
When we talk about server quorums, what I understand is that the concept is similar to STONITH in cluster i.e., we shoot the node that probably have issues/ make the bricks down preventing access at all. But I don't get how it calculates quorum.
My understanding:
In a distributed replicated volume,
1. All bricks in a replica set should have same data writes and hence, it is required to meet atleast 51% quorum on those replica sets. Now considering following 3x replica configuration:
ServerA,B,C,D,E,F-> brickA,B,C,D,E,F respectively and serverG without any brick in storage pool.
Scenario:
ServerA,B,F formed a partition i.e., they are isolated with other nodes in storage pool.
But serverA,B,C bricks are of same sub-volume, Hence if we consider quorum over sub-volumes, A and B meets quorum for it's only participating sub-volume and can serve the corresponding bricks. And the corresponding bricks on C should go down.
But when we consider quorum over storage pool, C,D,E,G meets quorum whereas A,B,F is not. Hence, bricks on A,B,F should fail. And for C, the quorum still will not me met for it's sub-volume. So, it will go to read only mode. Sub-volume on D and E should work normally.
So, with assumption that only sub-volume quorum is considered, we don't have any downtime on sub-volumes, but we have two partitions and if clients can access both, clients can still write and read on both the partitions separately and without data conflict. The split-brain problem arrives when some clients can access one partition and some other.
If quorum is considered for entire storage pool, then this split-brain will not be seen as the problem nodes will be dead.
And so why is it's not mandatory to enable server quorum to avoid this split-brain issue?
And I also assume that quorum percentage should be greater than 50%. There's any option to set custom percentage. Why is it required?
If all that is required is to kill the problem node partition (group) by identifying if it has the largest possible share (i.e. greater than 50), does the percentage really matter?
Thanks in advance!
Regards,
Jeevan.
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