Hyper-V Program Manager
A lot of people have asked me what happens “under the covers” when they delete a snapshot. Unfortunately, the answer is relatively complicated as there are multiple things that could happen.
As previously discussed – a snapshot is composed of a configuration file, saved state files and differencing disks (AVHDs). When you delete a snapshot the snapshot configuration file and saved state files are always deleted immediately – and the snapshot entry is removed from the user interface.
What happens to the AVHD depends on the situation of the snapshot and the state of the virtual machine. Here I have a virtual machine setup with a bunch of snapshots:
Let’s look at what would happen if I deleted various snapshots associated with this virtual machine:
This answer has probably raised some questions in your mind – so let me take a shot at answering some of them:
First: Thanks for your blog!! It’s always well written and very informative.
My questions are somewhat off-topic and pertain to the next version of Hyper-V (Windows Server 2008 R2)
1) I've seen max number of processors used by a Hyper-V Server has been increased, but I don't see anywhere talking about increasing a Vm's assigned processors. Will the max number of processors per VM be increased from 4? to perhaps 8?
2) Will Copy and Paste work like in a RDP window? (and like VMware's VM management tools: Vi Center, Vm Server, Vm Workstation). I know I can always open a RDP window, but I do software dev + testing and spend a lot of time in a bunch of different VMs and find the Hyper-V Manager's [Connect...] *much* more convenient. Except when I need to copy and paste text into (or out-of) the VM - it's a big a pain if I need to copy any more than a little text (typed text takes too long and truncates after a just a too few characters). And it's a pain to copy text from the VM to my host - I typically keep a .txt file on a network share to replace the missing copy and paste functionality :-(
Other than these two issues, I'm becoming a Hyper-V convert and slowing leaving behind the world of VmWare, ESX, VmWorkstationa and VmServer.
Also, are there any other interesting UI or performance enhancements to the next Hyper-V version? I’m waiting for the RC before I try out Windows Server 2008 R2.
Great article, however how could you "shrink" or "defrag" the disks after deleting some snapshots, I undestand if you delete from a sub-tree they would merge but I am having a hard time to makde this snapshots cleanup reflect in to the disk size.
I have several Images set for differents projects and the snapshots grow daily, I have exported them and obtained the same size of the images I have exported with less snapshots, as being said the snapshot deletion does not "flush" data from the disk, what would be the solution?
How can we detect that a merge is going on in the background?
Good article - thanks. I used it as the basis for my snapshot cleanup process...
"Also – many people would be confused by what “Merging a snapshot” actually meant"
If they are confused about that they shouldn't be in IT
What is the effect of running off a snapshot over a long period on a highly transactional server like Exchange. I can only imagine this will have a advers effect on normal operation and backup operation.
Well, it is still does not works as described here.
I had a single snapshot, afterthat the guest machine runned several days.
I did "delete snapshot" action, which is supposed to merge existing single AVHD with the original VHD.
The snapshot indeed disappeared from UI.
I shut down the machine, and started the machine again.
The machine did not merged AVHD with VHD, but happiliy continued to use AVHD.
Now i need to merge these VHDs manually.
I have a great respect to mircrosoft, but, MS folks, by yourselves you just push people to try VMWare, how one can resist ?
<rant>Why the heck cant it say merge when you merge a snapshot, and delete when you delete one. Everytime I have to go spend 20 mins googling to make sure I dont revert my virtual machine back to some worthless state it was in months or years ago... This entire system is semi retarted.. If you run on a snapshot for too long and dont have enough disk space to merge the snapshot into the vhd - it will basically crash your host!! Grrr...</rant>
I have my dev system on a VM. Over time I created several snapshots, some of them got rather large. I decided to reclaim the disk space by deleting the snapshots one at a time. The merge process is taking forever. Literally 12 to 18 hours for each avhdx involved. I noticed my disk activity is maxed out near 100% but the write speed is very low like 300 KB per second. I tested my disk and it rates at over 100 MB per second. What is causing the slowdown? Is there a wrong setting on the drive or something? Thx for any help you can give.
Your hardrive's SEQUENTIAL read/write performance may be 100MB/s which is common for a standard SATA desktop PC. Merging the AVHD back into the master is very much a RANDOM operation, and that type of performance for mechanical spinning drives is MUCH slower, commonly well under 1-2 MB/s. Please keep in mind that if you are reading from the AVHD and writing to VHD on the same drive, that will create more contention for hard drive performance. If this is a common scenario, I'd HIGHLY recommend a solid state drive as their random performance is a HUGE jump from mechanical drives.
In short, it's your hardware holding you back.