My Tape Drive Hates Me

[Richard Todd]

MATLEVAN at EKU.BITNET (Jerry LeVan) writes:
>#/etc/dump.bsd -T5.2 0cf /dev/rmt/tc1 /dev/dsk/c5d0s0
 Hmm, I should think it would detect the FS type automatically, but it's 
probably best to be paranoid about such things...

>...
>DUMP:estimated 67070 tape blocks on 0.87 tape(s).
>...
>  DUMP:51.09% done,finished in 0:29
>  DUMP:tape write error 38848 blocks into tape 1
>  DUMP:     tsize=76800, tenths=8, asize=38848
>  DUMP:NEEDS ATTENTION:Do you want to restart?: ("yes" or"no")

Yep, this looks like the "c" option isn't quite setting the parameters right. 
"Tape write error" under those conditions, at ~38.8K into the tape, sure 
looks suspiciously like it ran out of tape.  (One "block" according to dump
is 1K; as far as I know nowhere in the docs do they mention this).

   Hmm...an inspired guess: if "tsize" means the size dump thinks your Apple
tape drive has in blocks, then it looks like it's set to about twice as many
blocks as the tape actually has.  This is interesting, because in A/UX 1.1
dump blocks were only half as big (512 bytes).  Anyone want to bet that when
they upped the blocksize on dump they forgot to change the # of blocks that
"c" tells dump accordingly?  

>I tried the following command to explicitly set the size and
>block size.

>/etc/dump.bsd -T5.2 0bsf 8k 4800b /dev/rmt/tc1 /dev/dsk/c5d0s0

  Hmm.  I forgot to mention this in my first reply; as near as I can
tell the option of setting the tape size in terms of number of blocks
instead of number of feet of the (mythical, in the Mac world) 6250bpi
tape doesn't work :-(.  At least, I've never gotten it to work, so I
went back to the system I used on A/UX 1.1 (and is often used on BSD
machines with weird tape drives) i.e. do some creative lying on the
tape length values.

Give this dump command a try; I'm estimating the values here from the ones I
use on my TEAC 150M tape (i.e. multiplying by 38M/150M):

   /etc/dump.bsd -T5.2 0bsdf 8k 532 6250 /dev/rmt/tc1 /dev/dsk/c5d0s0

We're telling it that the tape is a 6250bpi 532-ft tape.  This is a lie, but
it should at least give dump roughly the right idea of how much can fit
on a tape.  Try it a number of times, adjusting the tape length (the "532"
in the above command) until it just fills a tape without going over.
Yes, this is a massive crock, but it does work.   Once you get the tape 
length value right, be sure to write it down or embed it in a shell script to
do your backups for you.  
--
Richard Todd	rmtodd at uokmax.ecn.uoknor.edu  rmtodd at chinet.chi.il.us
	rmtodd at servalan.uucp
"Cancelling a posted message means posting a cancel message."-Maarten Litmaath