Why Performing a 'Full' Format Doesn't Inflate a Thin Disk

On one of my recent courses a delegate noted that a full format doesn't inflate a thin disk. I've got an answer for him in this article. First, let's prove what he saw.

I created a new Windows virtual machine using the typical settings. The only change that I made was to select thin provisioned disks. The disk was 8GB in size and a typical VM has 1Gb RAM, totalling 9GB as shown. The VM has not been powered on at this point:

I connected a Windows CDROM and powered on the VM. Windows sees an 8GB disk on which I created a single partition.

Then next choice is how to format the disk. I chose the 'non-quick' format, which my delegate referred to as a full format:

If this wrote to every disk block then this format would take a while and we would expect to see the size of the thin disk increase. Once the format had finished the next phase is to copy temporary files over to the VM disk. I did not let this happen. I had to act quickly after taking this screenshot - I powered off the VM...

The final value of the disk size was as follows:

Great. So we've proved that the disk usage grows a little bit, but does not fill up the whole allocated disk space. Why?

This article (http://support.microsoft.com/?kbid=302686) from Microsoft explains what is happening when a Quick format and a 'Regular' format are performed. It's such a short technote that I don't think Microsoft will mind me posting it in it's entirety here:


When you choose to run a regular format on a volume, files are removed from the volume that you are formatting and the hard disk is scanned for bad sectors. The scan for bad sectors is responsible for the majority of the time that it takes to format a volume.


If you choose the Quick format option, format removes files from the partition, but does not scan the disk for bad sectors. Only use this option if your hard disk has been previously formatted and you are sure that your hard disk is not damaged.


The increase in disk usage is the creation of the partition table (very small) and the MFT. The format option performs no write operation on the actual disk blocks. This has some bearing from a security perspective as VMware defaults to a non-eagerzero format for the VMDK, meaning that previously written data may well be retrievable from inside the virtual machine... (Another article begging to be written there!)