MSFT finally talks snapshots in detail

Finally, after a long time of answering forums posts and my own posts here.

Ben Armstrong (or VirtualPC Guy fame) has posted a series of articles regarding snapshots and snapshot behavior.

Please, read and enjoy.  These are not going away any time soon.

Where are my snapshot files?

http://blogs.msdn.com/virtual_pc_guy/archive/2009/04/13/where-are-my-snapshot-files-hyper-v.aspx

What happens when I delete a snapshot?

http://blogs.msdn.com/virtual_pc_guy/archive/2009/04/15/what-happens-when-i-delete-a-snapshot-hyper-v.aspx

What happens when a snapshot is being merged?

http://blogs.msdn.com/virtual_pc_guy/archive/2009/04/16/what-happens-when-a-snapshot-is-being-merged-hyper-v.aspx

Why does it take so long to delete a virtual machine with snapshots?

http://blogs.msdn.com/virtual_pc_guy/archive/2009/04/20/why-does-it-take-so-long-to-delete-a-virtual-machine-with-snapshots-hyper-v.aspx

What happens if I start a virtual machine that is merging snapshot files?

http://blogs.msdn.com/virtual_pc_guy/archive/2009/04/21/what-happens-if-i-start-a-virtual-machine-that-is-merging-snapshot-files-hyper-v.aspx

Should virtual machine snapshots be used in production?

http://blogs.msdn.com/virtual_pc_guy/archive/2009/04/23/should-virtual-machine-snapshots-be-used-in-production-hyper-v.aspx

How to manually merge Hyper-V snapshots into a single VHD

Okay, so you have to recreate your VM configuration and you absolutely know that your VM had a snapshot at some time.

You also realize that if you just link to the base VHD that you will lose the current state of your VM - what do you do?

You manually merge your snapshots into your base VHD before you boot your VM. (I am assuming that you know how to connect to an existing VHD using the new VM wizard).
Merging of snapshots can be performed manually. This is achieved by:

1. On your Hyper-V host.Power off the Virtual Machine.
2. Make a copy of the VHD and its corresponding AVHD files.
3. Rename the AVHD extension to VHD.
4. Write down the order of the disks from youngest to oldest (the oldest should be the root VHD). You can do this by looking at the last modified time stamp on the origional AVHD files, find the one that last changed. And find the last one that changed before it.
5. In the Hyper-V manager, open the Edit Disk wizardBrowse to the youngest VHD in the chain, then choose 'reconnect' to point to the next youngest (the one that came before).
6. Open the Edit Disk wizard a second time and merge.
7. Then repeat the process until you have only a single VHD.

In a disaster case, you need to recover a copy of the root VHD prior to attaching it to a new VM and booting it (the act of booting it, modifies it)

Usually the most difficult part of this process is finding the last AVHD (differencing disk) in the chain.

The easiest way to do this is to find the configuration file for the VM.

Then open up that configuration file and locate the information for the virtual hard disk. In the screen shot below is the location of the current running state of the VM. The snapshot is a point in time to return to, the current running state is the "Now" and is contained in a differencing disk (AVHD) after a snapshot has been taken.


Now, find that AVHD file within the file system and rename it to VHD.

Now, go back tot he Hyper-V manager and open the "Edit Disk" wizard - Select the disk that you renamed above, and merge this disk into the one before it.

This process can be continued until all of the snapshots are merged back into a single VHD (the base VHD).

Hyper-V lost my vm and I connected to the vhd and lost everything..now I am panicking

I have read this story on the TechNet forums a few times. Something happened in Hyper-V, either a patch, or a temporary burp, or something else happened that caused a VM to be 'lost' from the Hyper-V console.

That might not be the worst thing.

Then a well meaning and informed administrator creates a new VM configuration attaching tot he existing VHD only to discover that a great deal of time on the life of the VM is gone.

What happened??

Most likely what happened was that your VM had a snapshot(s). If you claim that it didn't - think back, did you ever create one, then delete it? (and, you have not powered off the VM for an extended period of time to allow that deleted snapshot to be merged back into the base vhd..)

First of all the state of your VM may have been lost for good. The reason? - snapshots

I have blogged a bit about snapshots (here and here, and here) and they continue to cause confusion. I am guessing that the confusion comes from a lack of understanding of the underlying system.

The state of the VM is lost because the differencing disk integrity is broken. A differencing disk is a vhd that is chained onto an existing vhd. The first vhd (the base) stops receiving writes and the new vhd (the differencing disk) receives all of the writes to disk.the other item to know is that vhd is block based - therefore the differencing disk depends on the fact that the base disk does not change. If at any time you connect tot he base disk and modify it (even slightly) - the differencing disk chain is broken and there is not a tool (yet) that can put it back together.

Okay, you are now the better informed administrator - you made this mistake once and you tell yourself - never again!

This is a new post..

Snapshotting in Hyper-V does not equal VMware checkpointing.

The snapshotting concept under Hyper-V

For all of you familiar with ESX server and Virtual Center (or using a 3rd party product to do backups using checkpoints) you will have to think about things a little differently.

First of all.
Is using Hyper-V snapshots a good method to backup your VMs?

Answer: No. It isn't a backup method at all. It provides the ability to return to a defined point in time in the life of your VM. It is a very useful tool for testing application upgrades and service packs.

What are snapshots then (if they aren’t the same as a checkpoint)?

The best way that I have been able to describe snapshots is by using a timeline or traveling in time.
A snapshot is a moment in time that you can return to. And going right along with that - you can freely move forward in time, but moving backward in time will alter your future.

"Don't alter the timeline" - we have heard it hundreds of times (especially if you watch Star Trek Enterprise). Well, then you also hear the Vulcans state that time travel is impossible..

Snapshots in Hyper-V are all linked together, a single snapshot cannot stand alone (at least not without doing some tricks) as it is linked to its parent and so on.

The concept looks like this:

You begin with a base VHD. When a snapshot is taken an AVHD disk is created and the configuration of the VM is updated to use the AVHD as the current virtual hard disk.
(An AVHD is a snapshot specific differencing disk that is being used as the running point of a VM)

The AVHD disk is now the point where all system changes are written from this time forward - the base VHD is no longer being modified. And the AVHD is linked to (dependent upon) its parent disk. If you were to move one of these two files the VM is broken.

You can continue to spawn off additional snapshots. Each snapshot is linked to its parent in a linear (timeline) arrangement. They cannot link in a branched tree arrangement because that would create dead branches.

When you go back to a previous point in time (return to a snapshot) everything to the right of that point in time is destroyed (rendered unusable) because you altered the VM at a previous point.


Managing snapshots
Okay, what can I do with snapshots besides take them?

You can select a shapshot for a VM and “apply” it or “revert” to it.
This takes you to that moment in time and begins running the VM at that point in time. (You begin using the AVHD file that was created at that point in time).
“Revert” takes you back one step while “apply” can take you back many snapshots in one step.

Deleting a snapshot removes it from the tree.

You might wonder “if all of the snapshots are linked together how can I delete a snapshot out of the middle?”

This is because in the background Hyper-V is maintaining the integrity of your VM by combining the various states of your VM (or flattening the structure of your differencing disk timeline). If this didn’t happen your entire VM would be broken – that would be a serious bummer.

The merge process happens quietly in the background when a VM is powered off.
That is an important thing to remember – the VM must be powered off for all of the disks to be merged together.
This counts for anytime that snapshots are modified or merged into the Parent.

Delete a snapshot and its subtree.
This process deletes a snapshot and any other snapshots that are to the right of it in the timeline. The result is just that any snapshots taken to the right of the point in your timeline that you elected to perform this operation are all committed and merged into your current snapshot.

What if you want to “flatten” your tree and have everything written back to the base VHD?
Delete each of your snapshots one by one, even the current running one, and wait for the changes to be merged back into your base VHD. This can take a long time, and the VM needs to be powered down.

Deleting snapshots does not move you back in time, only applying a snapshot does that. Deleting snapshots merges your changes into another disk within the timeline.

Oh, and why don’t I have a bunch of screenshots of the GUI?
Because I figure that most Server Admins are smart enough to handle a GUI ;-)