windows - What is the different between sdelete -c and -z?

I've learned from this article to use sdelete as a step to shrink VDI file:

sdelete -c c:

But what does the -c ('Clean free space' as described by its help text) mean? Doesn't it write free space with zero? And what about -z(Zero free space)? What is the different between sdelete -c and -z?

I tried the step in the article mentioned before, but failed to reduce the size of the .vid file. Then I followed this superuser post, and changed the option -c to -z, and succeeded.

So it seems -z writes zeros to free space, and -c writes something else (random bits possibly).

Answer

I think this is a common mistake. Maybe the -z option was not available in earlier versions. SDelete 1.61 says

-z         Zero free space (good for virtual disk optimization)

Note that doing so will first expand your hard disk to the full size. Only in the shrinking process by VMWare it will become smaller again.

A quick test on my machine:

-z took 1:01 minutes
-c took 2:11 minutes

Which indicates that -z only writes simple zeros whereas -c is the military standard wiping DoD 5220.22-M as written on the SDelete website.

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