[Info-vax] VAX 6000 on Young Sheldon

[Tony Priborsky]

On Saturday, February 11, 2023 at 4:28:11 PM UTC-8, Arne Vajhøj wrote:
> On 2/11/2023 7:02 PM, Johnny Billquist wrote: 
> > On 2023-02-11 17:26, Arne Vajhøj wrote: 
> >> On 2/11/2023 11:18 AM, Tony Priborsky wrote: 
> >>> I'm pretty sure it didn't write a native VMS backup saveset - 
> >>> remember the CPU on the HSC was a PDP-11.   Logically the backup was 
> >>> the equivalent of VMS "BACKUP/PHYSICAL".  I seem to recall that at 
> >>> some point the VMS backup could restore (read) a HSC backup. 
> >> 
> >> So no CRC and no redundancy groups? 
> > 
> > Depends on how you mean that. Tapes have CRC on the data unless I 
> > remember wrong. Just like disks have.
> As I remember 9 track tapes then it was pure data blocks 
> and tape marks - no CRC. 
> 
> If mounted as labeled tape then some blocks were 
> considered VOL and HDR blocks, but that was an OS 
> feature not a drive feature. 
> 
> Arne

(and to previous) 

9 track tapes had a byte parity and a block CRC but no ECC.   The HSC backup depended on the tape error DETECTION but had no correction.  Given the shelf life of tape data (due to imprint) they were not intended as a long term backup but rather as a fast backup / replace disk / restore.   They were to be augmented by the slower OS based backups (remember, the HSC was used on both VMS (32bit) and PDP-20 (36 bit) machines.

The VMS backup was incredibly robust.   A VMS person - Kerbey Altmann as I recall - demonstrated the error handling capability by making a VMS backup saveset to magtape and then actually scraped the magnetic coating off a small piece of mylar (such that it affected at most one tape block, and for demo purposes did not span two blocks).  Then the restore operation kicked in to using the XOR block recovery and successfully "rode through".   You could see the tape "rocking" as it tried to find the missing data and then would re-sync and resume reading.    It was more than adequate to handle the hardware CRC errors that could happen on normal magtape due to wear etc.