[Info-vax] free shell accounts?

[Stan Radford]

On 2015-01-22, Phillip Helbig (undress to reply) <helbig at asclothestro.multivax.de> wrote:
> In article <m9q6li$5er$1 at speranza.aioe.org>, Stan Radford
><sradford at noemail.net> writes: 
>
>> 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.
>
> There are several possibilities, but probably most clusters have all 
> disks mounted on all nodes,

What does that mean actually? Does that mean they share physical disks
because they all have physical connections or something else? If it's
multiple boxes connected to one or more disk arrays that make sense. If not
I don't understand how it can work.

> is working on.  One or more nodes can go down, for planned or unplanned 
> reasons, and the cluster continues to exist.

As long as they're all physically attached to the same drives. But if
they're attached to network drives and the owning box goes down then there's
a problem.

> Of course, if a node goes down, the processes running on it will.  When 
> editing a file, then obviously you can't just continue editing 
> elsewhere.  All you have to do, though, is log in again (things can be 
> set up so that there is a virtual address for the cluster) and type 
> EDIT/RECOVER to get back to where you were when the crash occurred.

I don't understand how this can work until somebody explains the above
issues!

Stan