Why is shred's -z option twice as slow? I'm using `shred` to wipe a partition, and did some speed testing. My results were, in seconds: 6.4 using /dev/zero as the data source 6.5 shred's default raondom implementation 12.9 using shred's `-z` option Why is `-z` twice as slow? Here is the detail: # time shred -z -s 256M -n 1 /dev/sdc1 shred -z -s 256M -n 1 /dev/sdc1 0.14s user 0.05s system 1% cpu 12.919 total # time shred -s 256M -n 1 /dev/sdc1 shred -s 256M -n 1 /dev/sdc1 0.14s user 0.02s system 2% cpu 6.509 total # time shred --random-source=/dev/zero -s 256M -n 1 /dev/sdc1 shred --random-source=/dev/zero -s 256M -n 1 /dev/sdc1 0.00s user 0.06s system 0% cpu 6.419 total (I am using Linux Mint 20 on an external USB 2.0 magnetic drive formatted as ExFAT.)

Because it's doing two passes. One random pass followed by one zero pass.

If you want `shred` to zero and nothing else, it's `shred -n 0 -z`.

Add `-v` for verbosity and you'll see.