Pre-creation of saved state files under Virtual Server 2005 SP1 beta

A couple of people have noticed this relatively obscure addition to the Virtual Server 2005 SP1 beta release.  Basically what we now do is: when you launch a virtual machine under Virtual Server we will create a blank saved state file (.VSV) that is the size of the memory currently being used by the running virtual machine.  When you turn off the virtual machine we will delete this file – or if you do actually save state the virtual machine we will overwrite it.

The reason for this change is that we discovered that users could get into a very bad situation without this change if they ran out of space on their physical hard disk.  By using dynamic disks it is possible for a running virtual machine to try to expand its disk beyond the available space on the physical disk.  When this happens we pause the virtual machine and warn the user.  However – with not enough space left there is nothing the user could do at this stage except turn the virtual machine off and loose any data they were working on (shutting down the virtual machine or saving state would both require more disk space that was not available).  With a pre-created saved state file we can at least guarantee that the user will always be able to save the state of all running virtual machines – and then work on freeing up some space on the physical hard disk.