Hyper V Snapshot
- Hello all
My problem is this : I have an Exchange Server 2007 installed on a VM. I'm intending to change some settings, but before that I would like to take a snapshot just in case. My server has limited space and since I know that snapshots are taking much space from the HD, I want to be absloutly sure that I'll do this correctly. My Exchange server is something like 70GB of data and if I'll take a snapshot it will consume another 70GB of data. My question will be about the merging. Since I can't take off/shutdown my server because this is a very active server, how should I do the merge?
I don't understand why do you need to shutdown the server in order to do a merge. I assume that there is another solution for that, isn't there?
Please help me with this.
Thanks.
Eran
Answers
The alternative is: backups. Just as if the machine was not a vm.
A snapshot is not a backup.
Snapshot is a tool. It is designed to apply a change or update, then either revert or merge.
Yes, the merge creates downtime for the VM. This needs to be planned and scheduled.
As far as it consuming all of your storage.
You must be reading of the horror stories of folks that rely on snapshots as a type of backup, never change the default snapstho location off the system drive, and as a result end up consuming their system drive.
Then in a moment of panic, they begin the merge (already in a low available storage state) only to have the merge take an incredibly long time due to the already low resource state of the Host.
Saying all of this, I totally understand where you are coming from and your opinion. At the same time, in spite of the ease of use a hypervisor is not simply a point and click environment. Thought, planning, and understanding need to happen.
Does the use of snapshots ALWAYS end in a high disk use situation? No it does not.
Does it frequently end up this way? No it does not. However, there are cases where it can.
You are already beginning to think about this and understand this. So I anticipate that you will most likely never experience this storage issue that a few folks have gotten themsleves into (mainly because they don't understand how snapshots work - or they understand how VMware does it, and don't consider that Microsoft might do it differently).
Brian Ehlert (hopefully you have found this useful)- Marked As Answer byeran88 Sunday, June 14, 2009 3:20 PM
All Replies
- First of all:
Snapshots are not a copy of the VM. the amount of disk space that a snapshot can take varies on how much is changed after a snapshot is taken.
If you take a snapshot of a running VM the snapshot is far larger as all of the running memory space is written out as well.
Snapshots work through the use of differencing disks. The modifications to the VHD stops and a differencing disk is attachedat the time the snapshot is taken. When you return to the point in time the snapshot was taken (revert), the differencing disk is thrown away and a new one is created. Thus you start right where you left off - the same point in time.
In regards to power -off for merging.
Again, this is because of the differencing disk.
If you amount of change is small, then merge is relatively fast. If your amount fo change is large, then your merge will take longer. It all depends on how much change in the differencing disk need to be written to the VHD.
The running differencing disk cannot be merged to the parent, primarily because it is running, actively being read from and written to. And the intent is to have a solid VHD. If your disk corrupts you are not in a good state.
Lots of background can be found here:
http://itproctology.blogspot.com/search/label/snapshot
Brian Ehlert (hopefully you have found this useful) - First of all thank you for the quick reply. Second, as I understand and correct me if I'm wrong, you're saying that there is no way of merging the snapshot to the parent, without shutting down the server and let it merge, because of the differncing disk? If this is the case, Microsoft made it very difficult to take a snapshot before you do a change in the settings of the server. Because you now have to think twice before you do a snapshot, first because a change, as you say, can be small or large and without any warning consume all of your storage, whilst if you're thinking of doing a merge, you will have to shutdown the server and also pray that this will not also consume all of your storage.
This sounds very disturbing while thinking about using the virtualization enviroment of Microsoft.
Is there something you recommend or suggest of doing as a method?
Thanks again
Eran The alternative is: backups. Just as if the machine was not a vm.
A snapshot is not a backup.
Snapshot is a tool. It is designed to apply a change or update, then either revert or merge.
Yes, the merge creates downtime for the VM. This needs to be planned and scheduled.
As far as it consuming all of your storage.
You must be reading of the horror stories of folks that rely on snapshots as a type of backup, never change the default snapstho location off the system drive, and as a result end up consuming their system drive.
Then in a moment of panic, they begin the merge (already in a low available storage state) only to have the merge take an incredibly long time due to the already low resource state of the Host.
Saying all of this, I totally understand where you are coming from and your opinion. At the same time, in spite of the ease of use a hypervisor is not simply a point and click environment. Thought, planning, and understanding need to happen.
Does the use of snapshots ALWAYS end in a high disk use situation? No it does not.
Does it frequently end up this way? No it does not. However, there are cases where it can.
You are already beginning to think about this and understand this. So I anticipate that you will most likely never experience this storage issue that a few folks have gotten themsleves into (mainly because they don't understand how snapshots work - or they understand how VMware does it, and don't consider that Microsoft might do it differently).
Brian Ehlert (hopefully you have found this useful)- Marked As Answer byeran88 Sunday, June 14, 2009 3:20 PM
- Hello Eran,
I think you would be better off using backups for protection, rather than using snapshots in this case. This is especially true with Exchange, where disk IO and stability is critical. Snapshots move all following writes to an AVHD file, rather than the main VHD. They do not create a copy of the original VHD. To merge the snapshots back in you will have to shut down the server. If you want to protect your system I would perform a full backup of the virtual machine through Windows Server Backup or a VSS Hyper-V aware backup tool, as well as backup the Exchange databases through an Exchange 2007 aware backup tool.
Let me know if you have other questions.
Nathan Lasnoski
http://nathanlasnoski.spaces.live.com- Proposed As Answer byNathan Lasnoski Tuesday, June 16, 2009 6:08 PM
