[Info-vax] Backup TK50 tapes

[Paul Sture]

In article <kgjtbs$l2j$1 at dont-email.me>,
 David Froble <davef at tsoft-inc.com> wrote:

> Johnny Billquist wrote:
> > On 2013-02-25 21:27, glen herrmannsfeldt wrote:
> >> Paul Sture <nospam at sture.ch> wrote:
> >>> In article <kgfcuq$9f1$1 at Iltempo.Update.UU.SE>,
> >>> Johnny Billquist <bqt at softjar.se> wrote:
> >>
> >>>> Actually, I think for most hardware, the limit is actually 64k
> >>>> for one read.
> >>
> >> I have read/written 100K on unix 9track drives.
> > 
> > What system?
> > 
> >>> I believe that is correct.  However I purposely restricted VMS backup
> >>> tapes to a maximum of 32K because that meant I could copy savesets to
> >>> disk.
> >>
> >> Is that a VMS limit or DEC hardware limit?

It's an RMS limit.  Although BACKUP can write tape savesets of 64KB 
(actually 65,535 bytes so 64K minus a byte) you cannot COPY those files 
to disk, because the on disk limit for records is 32K.

COPYing backup savesets around can be a very useful time saver and on 
rare occasions I managed to get a saveset off a dying tape via COPY 
where BACKUP would fail with "too many errors".

It was the flexibility of being able to COPY savesets off tape which 
persuaded me to keep backup savesets at 32,256 bytes (512 bytes less 
than 32K), which is coincidentally the default size for savesets written 
to disk.

Tip: If you are suffering from brain fade and can't remember that 32256 
number, you can specify 32000 to BACKUP and it will automagically round 
up for you.
 
> > If the 32K really is a limit, I believe it would be in VMS, in treating 
> > a 16-bit number as signed here.
> > The controllers that I know by DEC have 16 bit fields for record lengths 
> > in hardware, but it is unsigned.
> > 
> >     Johnny
> > 
> 
> I'm going to take a guess here and state that the 32K limit was in VMS. 
>   Specifically, there is a VMS limit of 32K for string lengths.  The 
> string descriptors use 16 bits to specify the string size.  I'm going to 
> guess that string descriptors might have been used to specify buffers. 
> 16 bit length, 32 bit address ...

Going back to TK50 days, the VAXstation 2000 couldn't write backup 
savesets on tape with a blocksize greater than 16K and that was a 
hardware thing. I have a feeling it could read 32K block savesets though.

And since we are talking about the speeds of tapes here, the Cover 
Letter which came with TK70 drives had a section on increasing BACKUP 
block sizes from the default of 2048 and other tips to get the best 
performance.

-- 
Paul Sture