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,


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

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