Re: 3 or 4 nodes in a RAC?

Alan,

There is no 4-way gc waits. Any resource request will be accomplished with
in 3 hops. Anything above 3 nodes is as good as 3 nodes as there are at max
only 3 parties involved in Global Resource Directory. There is a master of a
resource and holder of the resource. If the requestor (say node 1) is
looking for a resource there are two possible scenarios depending on number
of nodes.

If there is two nodes, (Node 1 and Node 2), there are only one
 possibility.

Either node 1 may be Master and Holder.  In this case Node 2 requests the
access from Master (which is node 1), and Node 1 is also holder. You will
the grant with first hop. This is a two way grant or lock.

In three nodes, say node 1 is requesting access on Resource R1.  Here
two possibilities.

a. Node 2 is Master and Node 3 is Holder. (or Node 3 is Master and Node 2 is
Holder)
b. Node 2 is Master and Holder (or Node 3 is Master and Holder).

A. You request Master (Node 2 or 3). Master (Node 2 or 3) updates your
request to holder (Node 3 or Node 2). Based on the request type/hold type
you will get the block or grant. This is GC 3 way .
B. You request Master for access. If master is holder (like two node) you
get grant in first hop. This is like a two node cluster. GC Two way
messages.

This is same for whether you have 3 node or 30 nodes. Any request will
be addressed with in 3 hops.
Bottomline is , anything > 3 nodes is as simple (or complex) as 3 nodes.

Note:  This is way over simplified. But the logic is same :)

-Gopal;

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