What is tape block size of 0? From: benson@odi.com (Benson I. Margulies)

Tape devices are generally split into two categories: fixed block and
variable block. 1/4" tape is the fixed block, and 8mm is variable.

On a fixed block size device, the kernel always sends data to the device
in suitable block size lumps, and varying the size passed to write(2)
(e.g., via the bs option to dd) gives the kernel more data to stream.
On a variable block size device, the kernel writes to the device
whatever passed to it. On an 8mm, it had better be a multiple of 1024
to get efficient tape usage.

AIX has the World's Only Variable Block Size 1/4" tape drive. If you
use SMIT to set the block size to a nonzero value, AIX treats the device
as fixed block size, whether it is or not. By default, 8mm drives are
set to the same size as 1/4", 512 bytes. This is wasteful, but
otherwise mksysb and installp would fail.

If you set the block size to 0, the device is treated as variable block
size, and the size passed to write becomes the physical block size.
Then if you use a sensible block size to dd, all should be wonderful.

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