tar, mt backup BLOCKSIZES, variable & fixed question Seagate SDT224000N DDS-3 DAT

[Jonathan Wright]

Hello,

I'm having trouble understanding the tar & mt blocksize settings and 
operation.  From information around the internet, I think this is how it 
works but I want to run it by someone else who really knows:

If I run mt blocksize 1024, that sets the actual drive to write data in 
1024 byte blocks.  If I make an archive with tar with flag -b 20, tar will 
format and send 10,240 byte blocks to the tape drive and the tape drive 
will then subdivide those into 1024 byte blocks and put them onto the 
tape.  Working with larger blocksizes in tar better insures that the drive 
is always getting data, thereby making it not need to ever rewind to "catch 
up" if there is a delay in the host computer's data access.  However, I am 
getting inconsistent information on the variable blocksize setting in mt 
and don't know if I should use it or not.  One site I found said that 
variable blocksizes allow the drive to write blocks at a size that it 
thinks will be best for the data speed at hand, while another site said 
that it allows the drive to use the blocksize being sent to it from the 
backup application (making 10,240 byte blocks in the above tar 
example).  Who is right, or have I got this all mess up? :)

Also is there a preferred blocksize for DDS-3 DAT drives?

Thank You Kindly,

Jonathan

I'm using a Seagate SDT224000N DDS-3 DAT drive on FreeBSD 5.1-RELEASE.