As you know by now I’ve been building a high throughput, large volume Disk2Disk backup solution and that has been rather successful. At optimal speed we get 2TB/Hour per backup media server. As we currently have two, we can get to 4TB/hour at maximum throughput 
. Currently that is, we’ll see if more is possible. The solution, which I’ll blog about later, is based on the Dell Compellent hardware VSS provider (Dell Compellent Replay Manager 6.2.0.9), Windows Server 2012, CommVault 9.0 R2 and PowerVault storage a the target disks and as it’s working now is saving us many hundred of thousands of Euros compared to dedicated D2D backup appliances or solutions.
The entire process has been a fairly smooth one, which was a relief as decent hardware VSS providers are not easy to come by, based on our experience and that of many colleagues. So, once again the DELL Compellent choice is turning out to be a good one. We did have to fix one small issue along the way.
Whilst using the DELL Compellent Hardware VSS Provider with Commvault Simpana 9.0 R2 to do host level back of the virtual machines on Windows Server 2012 Hyper-V cluster nodes. We ran into a small issue. The backup run fine but we saw this error being logged:
Volume Shadow Copy Service error: Unexpected error querying for the IVssWriterCallback interface. hr = 0x80070005, Access is denied.
. This is often caused by incorrect security settings in either the writer or requestor process.Operation:
Gathering Writer DataContext:
Writer Class Id: {e8132975-6f93-4464-a53e-1050253ae220}
Writer Name: System Writer
Writer Instance ID: {3f4965d8-10ac-411b-bf6d-6a607f237775}
The description found here was not helpful in resolving this. This error is found all over the internet with just about any backup product. The possible causes and solutions that are suggested are as follows:
Change the Language for non-Unicode programs to English (United States)
This is not a solution in our case, which would have surprised me if it had been.
Eliminate the error condition by adding the access permissions for the domain account that the Compellent Replay Manager Service for Microsoft Servers (VSS) runs under to COM Security of the affected server
As the domain account used for this service is a member of the local administrators group that already has the permissions this would also have surprised me but we tested it anyway. It turns out this is not the cause either.
But for your information this is how it’s done:
You can eliminate the error condition by adding the access permissions for the Network Service account to the COM Security of the affected server. To add the access permissions for the Network Service account, do the following:
- From the Start Menu, select Run
The Run dialog opens.In the Open field, input dcomcnfg and click OK.
The Component Services dialog opens. - Expand Component Services, Computers, and My Computer.
Right-click My Computer and click Properties on the pop-up menu. - The My Computer Properties dialog opens.
- Click the COM Security tab.
Under Access Permission click Edit Default. - The Access Permissions dialog opens.
- From the Access Permissions dialog, add the DOMAINcompellentreplayservice account with Local Access & Remote access allowed (cluser => not just a local host).
- Close all open dialogs.
- Restart the computer.
The real cause & solution
According to Microsoft support this issue occurs when using a 3rd party backup program that utilizes Windows VSS (Volume Shadow Service) and has its own requestor. This is indeed the case here. “It looks like the requestor (the backup application) does not allow system writer to call back into their process and hence generates the error in the application log.” This sounds very plausible 
The fix:
- The following example grants access to the "DOMAINcompellentreplayservice" account.
- Click on Start, type regedit in the search box.
- On the Registry Editor window, navigate to: KEY_LOCAL_MACHINE>SYSTEM>CurrentControlSet>Services>VSS>VssAccessControl
- Add a DWORD value with the name: "DOMAINcompellentreplayservice" and set the value to “1”.
And voila: the error has gone when running backups 
I assume no responsibility if you do this in your environment but I can say that all this works perfectly in our CommVault Simpana 9.0 R2 setup whist backing up the virtual machines on our Hyper-V cluster nodes at the host level using the DELL Compellent hardware VSS provider. And yes this is Windows Server 2012, careful testing, planning helps when being an early adaptor.
).
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. I love it when a plan comes together!
