Very Computer

> >I'm trying to read a PDP-11 media tape that consists of backup data. I
> >tried using dd command in UNIX. What I have is only data COBOL data
> >structures. How do I know the block size of the tape.

> With some tape drivers you don't have to know, they'll
> transparently translate the blocksize-as-it-is-on-tape
> to the blocksize-as-requested-by-the-application for you.

> Failing that, if you're very lucky, a "mt status" on the tape
> drive will tell you.

> Otherwise you're likely to have to guess.  Some common sizes to try:
>   powers of 2 from 512 to 32768
>     (since 65536 doesn't fit in a 16-bit integer, larger values
>     are unlikely on a PDP-11 tape)
>   5120
>   10240
>   a power of 10 or twice a power of 10 (20, 200, ...) times the
>     size of the underlying COBOL data structure
>   256
>   1000
>   5000
>   10000
>   800
>   80
> Step through the list (I suggest something roughly in the same
> order as above, unless you have a reason to guess otherwise)
> until:  you both fail to get error reports from the tape driver;
> you see that the data does not have gaps at the end of a block;
> and you see that data is not truncated at the end of a block.

> --Ken Pizzini

> I'm not clear about what you mean by "PDP-11 backup command", as
> the hardware itself doesn't have a backup command.  I suppose,
> given the newsgroups you're posting to, that you mean some
> backup command that shipped with whatever version of Unix was
> being run on that machine?  If the "backup command" in question
> was "dump", then you should be able to use "restore" or
> "ufs-restore" to read it back, though it is possible you might
> need to feed the restore command the output of "dd conv=swab".

> In any event, you can get the data (contaminated with extraneous
> backup-structure data) with a simple "dd" command, unless the
> backup command does compression or encryption.  You will need
> some piece of software which knows about the format used by the
> backup command which wrote the tape in order to recover any
> structure (e.g., file boundaries) that may have been written, to
> clean out the "garbage" of the backup structures from the datat,
> and/or to undo the effects of any compression or encryption that
> may have been done.

>                 --Ken Pizzini

>OH...

>> I did check with Digital's ( now
>>Compaq) Engineers regarding the the issue. The feedback that i got is that I
>>need a VAX machine to read it.UNIX won't be able to read it directly. Anybody
>>else out there can confirm this, so that I can find some other alternatives.

>Well, I don't know enough about the RSX-11 backup command to
>make a definitive statement, but I am extremely dubious that
>their statement is strictly correct.  As long as the box has a
>tape drive which is physically able to read the tape (and which
>the OS has a driver for), it should only be a matter of having
>a piece software which understands the RSX-11 backup format to
>recover it.  However, it is true that it is extremely unlikely
>that you will find a unix system which ships with software
>that understands RSX-11 backup format, and I am unaware (not
>that I am likely to be aware) of any such software out on the
>'net, so you would probably have to write your own.

>So, from the point of view of "with off the shelf tools"
>they are probably right, but from the point of view of "it is
>fundamentally impossible" they are probably wrong.  In this
>vein, I will re-iterate one, possibly useless, piece of advice:
>>> In any event, you can get the data (contaminated with extraneous
>>> backup-structure data) with a simple "dd" command, unless the
>>> backup command does compression or encryption.

>               --Ken Pizzini

> On Sat, 30 Oct 1999 00:54:27 +0800,

> > What I really meant by PDP-11 backup command is that by
> >using PDP-11's RSX-11 OS backup command.

> OH...

> > I did check with Digital's ( now
> >Compaq) Engineers regarding the the issue. The feedback that i got is that I
> >need a VAX machine to read it.UNIX won't be able to read it directly. Anybody
> >else out there can confirm this, so that I can find some other alternatives.

> Well, I don't know enough about the RSX-11 backup command to
> make a definitive statement, but I am extremely dubious that
> their statement is strictly correct.  As long as the box has a
> tape drive which is physically able to read the tape (and which
> the OS has a driver for), it should only be a matter of having
> a piece software which understands the RSX-11 backup format to
> recover it.  However, it is true that it is extremely unlikely
> that you will find a unix system which ships with software
> that understands RSX-11 backup format, and I am unaware (not
> that I am likely to be aware) of any such software out on the
> 'net, so you would probably have to write your own.

> So, from the point of view of "with off the shelf tools"
> they are probably right, but from the point of view of "it is
> fundamentally impossible" they are probably wrong.  In this
> vein, I will re-iterate one, possibly useless, piece of advice:
> >> In any event, you can get the data (contaminated with extraneous
> >> backup-structure data) with a simple "dd" command, unless the
> >> backup command does compression or encryption.

>                 --Ken Pizzini