Virtual disk image (vdi)

Image structure

VirtualBox Virtual Disk Image (.vdi) files are structured in essentially the same way:

A header descriptor (512 bytes)
An image block map: If the (maximum) size of the virtual HDD is N MB, then this map is 4N bytes long.
Block alignment padding: The header format allows for padding between the image block map and the image blocks, and (as of version 1.6.2) the CreateVDI function adds padding after the map to ensure that the first image block begins on a 512byte sector boundary. Since the allocation unit on both NTFS and Ext3 file systems is 4096 bytes, you will therefore get slightly better performance (typically a few %) if you make your VDIs (1024N – 128) MByte long.
Up to N x 1MByte image blocks.