On Sun, Aug 15, 2004 at 02:42:27PM -0700, Aaron J. Grier wrote:
> 
> I'm quite not sure why.  I think it has to do with the block interface
> performing blocking which isn't desired or necessary with most
> tape-using tools, like dump and pax.
> 

Use the raw device when using tar/dump/pax because they are writing
their own data format.

Don't get "block interface" confused with tape blocking, they are
different things, tape blocking is used to increase performance by
writing large amounts of data to the tape at one time.  The idea being
that if you can feed enough data consistently then the tape will
stream which is much faster than it constantly doing a stop,
reposition, write - the larger writes gives the machine more breathing
space to get the next chunk of data ready to feed to the tape drive.

The block interface (/dev/st) will read and write data in blocks of
512 bytes (the standard block size), this is more useful on disk
drives and is, indeed, what the standard file systems work on (ever
noticed how newfs uses /dev/rxx but you mount /dev/xx?).
Theoretically (and some old OS'es supported it) you could put a file
system on a tape.  You need the right sort of tape drive (one that can
rewrite individual blocks) and it would be slow as something extremely
slow.  Having said that, I am reasonably sure that NetBSD does not
support putting a fs on a tape, but if you could then you would be
mounting /dev/st0... 

-- 
Brett Lymn