From: bill@bill.ab.umd.edu (Bill Bame)
Newsgroups: comp.os.vms
Subject: 2 node LAVC "problem"!
Keywords: VMS, LAVC
Date: 16 Nov 92 22:49:04 GMT
Reply-To: bill@office.ab.umd.edu
Organization: M.I.E.M.S.S.

Ok, I know this has been asked before (probably hundreds of times) here and
elsewhere, but I never thought I would be in this situation, so I didn't
bother to read the replys.  Before I go into detail, the question is:
"Is it possible to set up a two node cluster in such a way that crashing
either node does not reduce the number of votes below QUORUM".

I have a client who has replaced a LAVC made up of 4 overburdened
microvax IIs with a cluster of two 3100/80s.  That's the good news.

+--------+  +-------+  +-------+  +--------+
|System  |  |Common |  |Common |  |System  |
|Disk for+--+Data   |  |Data   +--+Disk for|
|VAX  #1 |  |Disk   |  |Disk   |  |VAX  #2 |
+----+---+  +-------+  +-------+  +---+----+
     |                                |
     | SCSI                      SCSI |
     +-------+                +-------+
             |                |
         +---+---+        +---+---+
         |VAX  #1|        |VAX  #2|
         +---+---+        +---+---+
             |    Ethernet    |
             +----------------+

The 3100/80s have separate system disks (VMS 5.5-1), but need to share queues
and disks (and tapes if DEC ever decides to support TMSCP-served SCSI tape
drives).  I suggested that they use DFS and DQS instead of VAXCLUSTER since
there are only two nodes - and since they don't share a system disk.  They
weren't comfortable with that idea, mostly because it would cost them extra
money, but also because they may want to add more nodes to the cluster in the
future.  I configured the two 3100/80s as a LAVC, and it runs just fine.  The
users seem VERY happy.  Now for the bad news...

There is a "perceived" problem caused by having a two node cluster (which I
warned them about in advance).  Because of the way QUORUM is calculated
[maximum of: (expected_votes+2)/2 and (total_votes+2)/2], the number of votes
drops below QUORUM if either node goes down.  Either node will boot
independently of the other since the QUORUM will be calculated using the
(expected_votes+2)/2 formula, but as soon as both nodes are up, the
(total_votes+2)/2 formula is used.  Let's say that EXPECTED_VOTES=1 and
VOTES=1 (on both nodes).  That means that QUORUM is 1 [(1+2)/2] for a single
node, but changes to 2 [(1+1+2)/2] once the second node has booted.  Simple
algebra shows that the (total_votes+2)/2 formula won't allow a node to drop
out of the cluster without "blocking activity" once both nodes have been booted.

Is there some trick (with a quorum disk perhaps) that will make this behavior
go away?

I realize that I should know this already (having set up dozens of clusters),
but I have never set up one with less than 3 nodes, and the manuals haven't
been much help.

Thanks!

-Bill
--
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William P. Bame                    | sneakernet: William P. Bame
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