Use of sfill -l ~; is it actually doing anything?

[Ralf Mardorf]

On Wed, 16 Oct 2019 21:01:46 +1100, Owen Thomas wrote:
>The disk is 900GB in capacity and according to the System Monitor 26GB
>or 2% (apparently) is being used.
>when should I expect it to finish?

The manual page mentions this:

"The secure data deletion process of sfill goes like this:

       *      1 pass with 0xff

       *      5 random passes. /dev/urandom is used for a secure RNG if
              available.

       *      27 passes with special values defined by Peter Gutmann.

       *      5 random passes. /dev/urandom is used for a secure RNG if
              available.

       afterwards as many temporary files as possible are generated to
       wipe the free inode space. After no more temporary files can be
       created, they are removed and sfill is finished." -
http://manpages.ubuntu.com/manpages/xenial/man1/sfill.1.html

Then do the calculation:

An example using GiB instead of GB, you need to do the calculation for
GB yourself.

(1pass+5passes+27passes+5passes)*(900GiB capacity-26GiB used)=33212GiB

33212GiB/1024=32.43TiB

How long it takes to write around 32 TiB depends on the used hardware
and the I/O handling of the software. Likely the theoretical B/s max.
value has got nothing to do with realistic averaged values.

However, perhaps you already have used gparted to resize and move
partitions on you 900 GiB sized HDD. When doing so data gets copied,
IOW read and written, perhaps tens or hundreds of GiB. This could take
very long for those GiB, so writing tens of TiB takes many times over
very long.

Keep in mind you are most likely overwriting the data multiple times for
esoteric reasons only.