On Thu, 13 Apr 2000, Chuck McManis wrote:

> >> But apart from that, dd is *not* a usable tool.
> >> This is a typical example when Unix tools fail miserably. Tapes have
> >> records, of perhaps different lengths. You need to preserve this
> >> meta-information when copying tapes, or the copy will not actually be the
> >> same as the original, even though you have the same bytes on both.
> 
> I don't know of any tape that varies record lengths within a single _file_
> on the tape. Typically tape formats are :
> 
> 	<FM> record <RM> record <RM> record <FM> record_type2 <RM> ... <FM><FM>

You are Unix-damaged. :-)

Typical example for variable length records are ANSI format tapes.

> If anyone has a counter example please let me know! There may be variable
> length records within the fixed length records (witness tar archives) but
> records between two file marks are the same size. dd should be able to
> figure this out. If I had a working copy of this tape I could help figure
> it out :-)

But apart from the fact that ANSI tapes have varying record sizes, just
because dd manage to read records correctly, it can be non-trivial to know
what that record size was, so that you can use dd to output to another
file. dd default record size is 512, which might not be what you want.

Oh, if you want additional examples of tapes that don't do fixed record
sizes. VMS BACKUP, RSX BRU, just about any OS plying with tapes, except
for Unix. :-)

Oh, and DEC was pretty fond of ANSI tapes, and this is a DEC tape we're
talking about, so I wouldn't be totally surprised if it had some ANSI
headers on it.

(I guess people pretty much ignore ANSI tapes nowadays, since Unix don't
want to handle tapes that much at all, but I think they are a pretty good
thing anyway. :-)

	Johnny

Johnny Billquist                  || "I'm on a bus
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