[Simh] VAX Tape Emulation?

[Mark Pizzolato]

On Tuesday, February 20, 2018 at 12:25 PM, Zane Healy wrote:
> > On Feb 19, 2018, at 10:42 PM, Mark Pizzolato <Mark at infocomm.com>
> wrote:
> >
> > On Monday, February 19, 2018 at 9:55 PM, Zane Healy wrote:
> >> I’d posted about this nearly a month ago, and have made some real
> >> progress this weekend.
> >>
> >> Configuration:
> >> MONK
> >> OpenVMS 8.3/Alpha
> >> Compaq XP1000/667
> >>
> >> RENNY
> >> OpenVMS 7.3/VAX
> >> SIMH MicroVAX3900
> >>
> >> I’m backing MONK up over the Cluster to a “tape drive” on RENNY.
> >>
> >> First, Ubuntu includes a package for SIMH V3.8-1.  If you’re just
> >> using disk and network, it seems to be just fine.  If you want to
> >> write tapes, it seems fine.  If you want to read tapes, upgrade, as
> >> it appears to crash OpenVMS V7.3, at least when messing with “tapes"
> >> the size I’m creating.  Hopefully the crashing isn’t a clustering
> >> issue, I still need to test that.  I downloaded the latest SIMH sources, and
> built them today, and I appear to be able to read tapes without crashing.
> >> At least it doesn’t crash when reading from the SIMH VAX, I still
> >> need to try reading one over the cluster on my Alpha.
> > [...]
> >> One stupid question, I assume that SIMH emulates the network
> >> interface at 10Mbit.  Is there any way to speed that up?  Granted,
> >> all my real VAXen are limited to 10Mbit, but a faster link in SIMH would be
> nice.
> >
> > The network interface is not artificially limited to 10Mbit.  Years
> > ago, I measured some 25Mbit on my desktop system.  I suggest that you
> > run with the latest code from https://github.com/simh/simh.  Simh
> > v3.8-1 was from a long time ago, and significant changes have been
> > made since then.
> 
> I grabbed the latest copy of the source from GitHub yesterday.  I ran another
> backup last night using the current SIMH, and it was the first to die before
> finishing, but there could have been other issues.  I’m trying again, as that was
> also my first attempt and running it with the /LIST option.
> 
> As noted, the latest SIMH was needed to successfully read the tape (I simply
> did a listing).
> 
> It’s hard to judge the speed accurately, due to tape Metadata, but I’m seeing
> the ‘virtual tape’ be created at about 510KB/sec.  That is faster than what I was
> previously seeing.  I wouldn’t be surprised if part of the bottleneck is the
> virtual tape drive.

I would actually attribute the 'improved' behavior to the fact that tape I/O to TQ 
and XQ devices is asynchronous in the latest code.

I would seriously consider Mr Baker's suggestion of backing up to simh disks.  Not to save sets, but to merely using VMS backup to perform BACKUP/Image to RQ units attached to disk container files.  Thus allowing any recovery you might need to have random access to the original file system without having to dig through a save set sequentially.

An industrious fellow might also want to manage the RQ media setup and change activities automatically from the AXP system with a program that does a TCP connect to a simh Remote Console.

- Mark