What is the principle of VirtualBox's dynamic size hard disk?

Do you know TrueCrypt Project?

It makes a container file and formats it.

The container file will be seen as like a volume.

An we do I/O to the volume, the I/O will be crypted.

By the way, can the container fi开发者_如何学Pythonle size be flexible like virtualbox's dynamic hard disk(.vdi file)?

I made a crypt driver like the TrueCrypt.

But my container file always hold disk size even if I don't write any data yet.

How can I implement this feature in Windows.

What is the principle.

Thanks.

---

The principle is simple - actual space for virtualbox's hard disk is allocated when something is written there. There can also be sparse files used.

TrueCrypt, as I know, encrypts all disk contents, performing writing on each sector of the disk (also to make encryption stronger), so afaik it's not possible to have a dynamically resizable truecrypt disk.

Normally file systems should support dynamic sizing in order for this feature to work efficiently. However, for regular file systems (FAT, NTFS, extX etc.) there's no need to be shrinkable as they were designed to occupy space, dedicated for them on hard disk.

Our SolFS product offers a file system that was designed with resizing in mind and it also supports on-the-fly encryption and both can be used independently of each other. And OS edition of SolFS lets you create a virtual disk, visible using drive letter.