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 devicewhatever 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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