block_size=$(bup-cfg-py -c \
"import os; print(getattr(os.stat('.'), 'st_blksize', 0)) or $mb * 3") \
|| exit $?
-data_size=$((block_size * 10))
+data_size=$((block_size * 20))
readonly block_size data_size
WVPASS dd if=/dev/zero of=test-sparse-probe seek="$data_size" bs=1 count=1