[Simh] HSC vs UDA/QDA

[Johnny Billquist]

On 2017-03-09 02:26, Paul Koning wrote:
>
>> On Mar 8, 2017, at 7:44 PM, Johnny Billquist <bqt at softjar.se> wrote:
>>
>> On 2017-03-08 22:15, Bob Supnik wrote:
>>> The HSC family offered a superset of capabilities compared to the
>>> UDA50/QDA50. In particular,
>>>
>>> - tape as well as disk support (TMSCP as well as MSCP);
>>> - controller-based disk to tape backups and tape to disk restores;
>>> - controller-based disk to disk duplication;
>>> - controller-based volume shadowing (RAID 1).
>>>
>>> UDA50/QDA50 did not support tape drives, disk duplication, or
>>> controller-based volume shadowing.
>>>
>>> HSC supported some data caching; UDA50/QDA50 did not.
>>
>> Oh. I didnät mean to imply that the HSC was just the same as an UDA. But the MSCP protocol as such is the same between them. Shadowing, local disk copying, caching and so on, are just things a controller can do without the host are even aware of it happening.
>> But since we're talking emulation, the actual disks now might be doing even more of that than an HSC ever could. It's not really something that makes much sense to emulate.
>
> I think the significant different for emulation, as opposed for the real hardware, is that CI is a multi-access network (like Ethernet).  All the hosts can see all the disks, and in addition the hosts have peer to peer communication.  VAXclusters use both of these things.  You can of course do them via LAVC (same services but over Ethernet).  With CI emulation you get a second way.  That enables running clusters with VMS versions predating LAVC, if that is interesting to anyone.

Yes.

However, to get that working you will either need to run several 
emulated machines under the same simh instance, or have the CI run 
outside of the simh framework, so that several simh instances can 
communicate with it. While possible, this could turn complicated.
If you only do CI within CI, with the limit to one machine, then all 
that is lost, and you end up with the same as a local MSCP and TMSCP 
controller, with just a different transport layer you need to implement.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol