[Info-vax] Wide area cluster, metro area network, seeking info

[dthi...@gmail.com]

On Tuesday, June 8, 2021 at 7:12:31 PM UTC-4, Arne Vajhøj wrote:
> On 6/8/2021 6:28 PM, Rich Jordan wrote: 
> > We are looking at the possibility of putting VMS boxes in two 
> > locations, with Integrity boxes running VSI VMS. This is the very 
> > beginning of the research on the possibility of clustering those two 
> > servers instead of just having them networked. Probably have to be 
> > master/slave since only two nodes and no shared storage.
> You mean node A 2 votes and node B 1 vote?
> > After reviewing the various cluster docs, they seem to be focused on 
> > older technologies like SoNET and DS3 using FDDI bridges (which would 
> > allow shared storage). The prospect has a metropolitan area network 
> > but I do not have any specs on that as yet. 
> > 
> > Are there available docs relevant to running a distributed VMS 
> > cluster over a metro area network or fast/big enough VPN tunnel? Or 
> > is that just the straight cluster over IP configuration in the docs 
> > (which we've never used) that we need to concentrate on?
> First you need to find out what protocols they can support. Most 
> likely IP only. That means you need to look at IP. 
> 
> Then you need to ask a lot of questions about latency. Average, 
> maximum, distribution etc.. Because high latency can kill the 
> project. 
> 
> Arne


This might a good time to call in VSI or <another OpenVMS consulting group> to consult about creating a successful and stable wide area cluster. :-)

With Integrity systems, you basically have only one cluster communications channel: Ethernet. So you have a choice of a LAN Cluster (LAVC) or a WAN Cluster (Cluster-over-IP).

With the LAN Cluster, you have to _bridge_ the sites together with fairly expensive network equipment or create a poor-man's VPN tunnel, which then means your cluster has a secondary dependency upon the stability of the tunneling/bridging systems. The MAN (Metropolitan Area Network) technologies can fit into the LAN cluster category, although they don't have to. With Cluster-over-IP, you can use existing internet routing between the two sites, although you do have to open the SCS communications port(s) in your firewalls. A MAN is still an option for Cluster-over-IP, although a rather expensive one compared to the cost of a standard ISP Point-Of-Presence.

As Arne pointed out, latency can be a big problem for a wide area cluster, regardless of whether you choose LAN or WAN. The cluster design book comments about factoring the latency between your sites when setting cluster parameters. HPE/VSI recommended keeping the distance between clustered sites to under 500 miles due to latency, although OpenVMS engineering at HPE India did forge a planet-wide OpenVMS Cluster-over-IP centered in Banglore, India. They noted that the cluster parameters had to be tweaked heavily to support the excessive distance latency.

Regarding cluster votes and quorum: You might find that purchasing a cheap second-hand Integrity located at a third site to provide a tie-breaking vote could enhance your cluster stability. :-)


David