[blindm]

> does not necessarily mean you're actually writing out 20GB to the disk.

You mean like preallocation? I think Virtualbox now does that. In the past it didn't though, it just kept writing a bunch of zeroes to the drive until it reached 20GB.

[mmis1000]

Or probably the filesystem decides to do it? The Refs will just ate adjacent 0 and assuming you want a fallocate here.