[Simh] HSC vs UDA/QDA

[Paul Koning]

> 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.

	paul