create really big files from shell (mac here, linux similar)

$ time dd if=/dev/urandom of=testfile bs=1024 count=$((1024 * 1024))
1048576+0 records in
1048576+0 records out
1073741824 bytes transferred in 158.475750 secs (6775433 bytes/sec)
real    2m38.479s
user    0m1.718s
sys     2m29.530s

$ time dd if=/dev/zero of=testfile bs=1024 count=1 seek=$((1024 * 1024))
1+0 records in
1+0 records out
1024 bytes transferred in 0.000075 secs (13634817 bytes/sec)
real    0m17.936s
user    0m0.001s
sys     0m0.844s