On Tue, Jul 11, 2017 at 7:17 AM, TranceWorldLogic . <tranceworldlogic@gmail.com> wrote:

Hi,

I was trying to setup hosted engine on 2 host machine but it not allowing to setup hosted engine on 2 node gluster file system.

I have also modified vdsm.com to allow 2 replica count.

But latter I found in code it is hard coded to 3 as shown below:
------------------- ------------------------------- ------------------------------ ---------------
src/plugins/gr-he-setup/storage/nfs.py

        if replicaCount != '3':
            raise RuntimeError(
                _(
                    'GlusterFS Volume is not using replica 3'
                )
            )
------------------- ------------------------------- ------------------------------ ---------------

Can I get to know the reason to hard coded to 3 ?
from online I got to know that split brain issue not get resolve automatically using 2 node gluster setup. But please let me know more (why 3 ? why not 2 or 4 ?)

Simply because two is even and three is odd.

In a split condition over only two nodes, if node A says A and nodes B says B you cannot decide if the truth is A or B.

With three nodes, if node says A but nodes B AND nodes C says B, the truth is B by design.

That's why you need three nodes with at least 2 of them up and working.

If you want to save something on storage side you could evaluate two regular nodes plus an arbiter one. 

 

If I am missing something please let me know.

Also want to understand, can quorum can solve 2 node gluster issue (split-brain) ?

Thanks,

~Rohit

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