[Info-vax] Copying VMS SaveSet Under Windows

[glen herrmannsfeldt]

Johnny Billquist <bqt at softjar.se> wrote:
(snip, I wrote)

>> Even more, if you use dd with the bs option, for a tape to tape
>> copy, it will preserve the block size.  That is different than
>> the ibs and obs options, which reblock the data to the specified
>> obs block size.

>> With bs=(large value), dd reads one block, writes one block,
>> reads the next, writes that, etc.
 
> It will not "preserve" the blocksize. You will just not loose any data, 
> but the block size information is lost.

When dd runs with the bs= option, it reads a block (up to blocksize),
then writes that block (of whatever size) to tape.  Repeat until EOF.
The blocks written have the same size as the blocks read, unless one
is bigger than bs.  If you specify both ibs= and obs=, then the
data is reblocked to the obs size, and block information is lost.
 
> If you want to preserve the blocksize, so that you can recreate the tape 
> at a later date, you need to write your own program under Unix.

If you dd to disk, then the block information is lost.  That comment
was for tape to tape dd.  

For tape to disk to tape, there are a few different "virtual tape" 
formats that keep the block information in a disk file, to be 
converted either back to real tape, or read by an emulated processor.
 
> Feel free to ping me if you want. I have written such a beast (as I'm 
> sure many others have), and I've published it in the past, and can sent 
> it to anyone who needs it.

-- glen