On 2015-06-10 09:34, Mark J. Blair wrote:
I was looking at a couple of documents describing the Pertec tape interface; 
the manual for my Kennedy 9610 tape drive, and a nice reference by a fellow 
with a rather familiar name:

     http://www.sydex.com/pertec.html

According to my Kennedy manual, issuing a read command causes the drive to 
return one block of data. I can see how that would be used in block-oriented 
applications in which blocks may be randomly read, written and re-written on 
the tape. But most of my magtape experience has been using the tapes in a 
streaming mode, such as when reading/writing one or more tar archives separated 
by file marks.

Yes. Tapes pretty much always have blocks...

When writing a tar archive on a magtape from a Unix system, is the archive 
written as a sequence of fixed-size blocks? Or is the entire tar archive 
effectively written as one continuous block which must be streamed with no 
repositioning?

Yes, tar uses a fixed block size. You can actually tell it what block size to use.
Most versions of tar I know use the -b switch for this.

I'm curious because I'm daydreaming about how to build a tape drive interface 
controller, and I wonder whether it might need to potentially stream an entire 
tape in one go vs. being able to safely assume some maximal block size.

You can pretty much assume that no tape uses larger blocks than 64K. But you have to expect blocks of pretty much any size below that.

        Johnny

--
Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: b...@softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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