[Info-vax] free shell accounts?

[Simon Clubley]

On 2015-01-22, David Froble <davef at tsoft-inc.com> wrote:
> Stan Radford wrote:
>> 
>> I don't understand what a cluster does. If they don't have shared disks
>> somewhere wouldn't they have to have multiple copies of everything? How does
>> a cluster still remain usable if you are editing a file and the machine the
>> file lives on fails? I can see for serving applications a cluster would be
>> great but I don't understand how it helps development users. And even that
>> would seem like it would take a lot of planning and wouldn't just automatically
>> "work" because of the need for shared storage somewhere.
>> 
>
> The concept of a VMS cluster is "shared everything".  There can be 
> various implementations.
>
> For best versitility, each computer (node) would have a connection to 
> every device.  Thus, if a particular node in the cluster goes down, all 
> other nodes still have access to all devices.
>

I wonder if what Stan is missing is the concept of the DLM and what that
enables.

Stan, in a shared everything configuration, the disks do not belong to
a specific machine but all VMS machines in the cluster can all write to
the same disk (and update the filesystem structures on that disk) at the
same time.

This is made possible in VMS land by the use of a Distributed Lock Manager
(or DLM for short). The DLM is a cluster wide common database of locks
and allows multiple VMS systems physical access to the same disks along a
common hardware path as the DLM allows these multiple VMS systems to
automatically coordinate their activities with each other.

This includes a machine leaving (or joining) the cluster which is also
handled automatically by an automatic procedure known as a cluster state
transition. Once the state transition is complete, the remaining nodes
just carry on from where they were.

> Now, it is also possible for devices to be connected to a single node, 
> and that node can share them with the cluster, but such devices are lost 
> to the cluster if that node goes down.
>
> With today's technology, some people use SANs, and then each node in the 
> cluster is basically a "compute device".  As long as one node and the 
> SAN is up, the cluster exists.
>
> I may not be the best source of cluster information, and I'm pretty sure 
> that except for an brief overall description a newsgroup post is not the 
> best place to learn about cluster details.

I would agree with the latter bit about learning cluster details. :-)
I think Stan has some reading ahead. :-)

Simon.

-- 
Simon Clubley, clubley at remove_me.eisner.decus.org-Earth.UFP
Microsoft: Bringing you 1980s technology to a 21st century world