[Info-vax] Backup TK50 tapes

[glen herrmannsfeldt]

Dennis Boone <drb at ihatespam.msu.edu> wrote:

(snip)

> You continue to fail to get it.  It's not about how "low-level" the
> tool is.  The blocks aren't necessarily all the same size.  You may
> not _know_ the size, even if they are.  The next guy who has to pick
> up your image _probably_ doesn't know.  The size of the blocks matters
> in many cases.  If you use dd, you THROW AWAY this information, and
> save just a stream of bytes.  Don't use dd to image tapes.  Don't.
> Eh eh eh, I see you thinking about doing it anyway.  STOP.  Use a
> tool which saves this data.

The unix read() and write() system calls don't throw away the length
information, but the disk file systems do. 

Now, if you use ibs= and obs= then dd will reblock the data, even if
you didn't want it to, but also if you did.

But if you use bs= for tape to tape, dd does read for each block,
and then write() for the block.
 
> If you're going to use the image with a simulator, it probably wants
> .tap format or similar anyway.  A .tap image preserves the block sizes.
> There are plenty of tools for building such images.  Use one.

Yes .tap or one of the other formats is often a good choice.

-- glen