Is your VMware snapshot application consistent ??

Update: August 22, 2010

Taking snapshots of your VMware virtual machine seems a safe, efficient, simple way of securing the virtual machine and its application data. Quite a few back-up solutions like EMC Replication Manager, Dell Equallogic Auto-Snapshot/VMware Edition and Veeam Backup and Replication uses this snapshot technique.

However, if your virtual machine is running a transactional database (Active Directory, SQL, Exchange, Sharepoint, Oracle) you might find out while restoring the virtual machine the database is not consistent.

This is because VMware vSphere does not fully support VSS-writers if you are running Windows Server 2008 guests.! For an overview of guest operating systems and if a snapshot is only file system consistent or application consistent, see this VMware document.

First some info on back-ups.

To create a back-up of virtual machine there are two methodes:

1. Using an agent in the virtual machine. This is the traditional way of making back-ups also done on physical servers. To back-up applications using a transactional database, you need to install an agent which is application aware. The main purpose of the agent is to pauze the database, put it in a consistent state, have the back-up run, delete transaction file and release the database for writes. This is called a guest level back-up.

2. Using the snapshot function of VMware.In this situation a snapshot is made of the virtual machine. The original VMDK file is tranfered to another storage location, and the snapshot is deleted. This is called an image level back-up.

As said, VMware is not able to use VSS to quiesce the app, use VSS to take a snapshot of the volume(s) the app resides on via VSS and then back up that snapshot, then release the app if your guest is running Windows Server 2008 or Windows Server 2000. This is true for Windows 2008 servers running on vSphere 4.0.

For vSphere 4.1, VMware supports application consistent quiescing for guests running Windows Server 2008! This is if the VM has been created on ESX 4.1 hosts. If the VM has been created on vSphere 4.0, you will need to edit the properties of the VM to enable application consistent quiescing. Read more about this at my good friend’s blogsite

Some back-up software providers offering image level back-up solution install special agents into the virtual machine to talk to the VSS writer to solve this problem. Veeam Back-up and Replication is one of those solutions. VMware Data Recovery does not!

Read all about this on an article at Backup central called Hyper-V ahead of VMware in the backup race ‘ written by W. Curtis Preston

And also read the blogpost on the VeeamMeUp website. Very interesting articles and links on making application consistent snapshots for your back-up’s.

At Vmware Partner Exchange 2010 one of the session was about called “How VMware Leverages Microsoft Volume Shadow Services for Virtual Machine Snapshots”. A live blog of this session can be found on the site of Scott Lowe. It gives some interesting info on the proces of making a snapshot.


About Marcel van den Berg
I am a technical consultant with a strong focus on server virtualization, desktop virtualization, cloud computing and business continuity/disaster recovery.

4 Responses to Is your VMware snapshot application consistent ??

  1. VMDoug says:

    Thanks for the post on Veeam and VSS! There’s been a lot of talk on the blogosphere lately about VMware’s VSS implementation and I’m glad to see someone who finally go it right!

  2. Pingback: Application consistent quiescing and VDR « vKnowledge

  3. I have confirmed that 4.1 now supports application consistent snapshots. What I _haven’t_ confirmed is whether or not they are still using VSS_BT_COPY instead of VSS_BT_FULL. The former does not tell the app they’ve been backed up, and the latter does (so it can do things like truncate transaction logs). MAN I wish I was going to VMworld…

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