#microsoft virtualization

How It All Works

As mentioned in our previous episode, today we are focusing on the use of DiskShadow to create a snapshot of your hard drive and and everything on it. The cool think about DiskShadow is its name - DISKSHADOW - just sounds like the horse of a mythic warrior or something. Anyhow the OTHER cool thing about DiskShadow is that it uses the Volume Shadow Copy Service or VSS to create that snapshot.

Why that matters is that the VSS is able to look at the programs using that disk at the moment of snapshot and, once they are VSS-aware, cause those programs to bring their data to a consistent state and stop making changes for just a millisecond. This is long enough for a snapshot of that program and its data to be taken.

This is equivalent to asking the program to smile and say cheese during the snapshot. The technical term of it is Quiesce. The program is quiesced or put to sleep during the snapshot.

The reason for this period of quiescence becomes readily apparent when you try to restore data from that snapshot. If you try to copy a data which is in use you are likely to get corrupted or inconsistent data and trust me, ain't nothing like restoring a "good backup" to recover a critical server only to discover the backup is not as good as you thought.

If DiskShadow puts the program to sleep via VSS however, no data is written during the snapshot and furthermore the program ensures its files are nice and tidy so that restores are largely painless. Hyper-V is VSS aware and obediently goes to sleep for the millisecond when the snapshot is taken.

The goal would be to:

1. Automatically quiesce the virtual machine for a few milliseconds (not even enough for user to notice)

2. Take a snapshot of the consistent vhd files.

3. Bring the virtual machine back online.

4. Copy the VHD files from the snapshot to another location (Remember, when you work with the snapshot, you are not actually affecting the real files so users can continue to use those files with virtually no interruption while the copy operation occurs)

5. Delete the snapshot. They don't actually take up a lot of space but they can add up over time.

You should ensure your virtual host has enough RAM, processing power and network bandwidth (if copying to a remote location) to handle the copy operation while still providing resources for the current virtual machines.