Although Microsoft would like everyone to be backing up their data to Azure from DPM, the reality is that most organisations still feel more comfortable with the traditional backup to tape. I’m sure this will change over time, but this is where most people are at the moment.

This means that DPM has to be able to protect to tape, which it can do, but typically these days a tape library has capacity to backup a lot more data than a single DPM can protect.  In this scenario, you would want to make full use of your tape library, and you wouldn’t want to manage a tape library for every DPM server in your environment.

The solution to this is to utilise tape library sharing in DPM, which simply allows multiple DPM servers to share a tape a library.  One DPM server will act as a “library server” to control the medium changer and send instructions to the library, and all the other DPM servers will act as “library clients”.

 

DPM Library Sharing

 

Follow this guide if you need to set up tape library sharing, https://technet.microsoft.com/en-us/library/hh758124.aspx.

Tape library sharing is a great feature and allows maximum usage of a tape library, or even a virtual tape library, without changing the restore methods with DPM itself.  However, I have found that troubleshooting issues with tape library sharing after it has been set up, should any be experienced, very difficult.

The issue is that tape library sharing is a rather complex beast.  There are multiple SQL components involved, as well as an array of SQL tables and stored procedures, plus DTC, DCOM, and WMI… All this can make it extremely difficult to troubleshoot a fault, and even more difficult to resolve.

Usually though, when an issues arises with Microsoft product, you head straight to Bing (other search engines available) and see what information is already published regarding the issue, and usually end up with a resolution, or at least guidance for a resolution.  Unfortunately, if this approach is attempted with DPM tape library sharing, very little information is returned.  There are plenty of posts providing high level information on how components hook together for library sharing, but nothing (or very little) in detail about how it actually works.

After experiencing numerous tape library issues, I started to find this lack of information, and lack of published support information very frustrating, as time after time I was having to raise calls with Microsoft support to get a resolution.  What made things even more annoying was that all the tape issues experienced had occurred because of some kind of unknown issue in the product, or a bad UR release.  So, I decided to raise my concerns over the lack of detail and support articles with Microsoft, and I must admit to being rather nicely surprised with the honesty of the response…

Basically, I was advised that Microsoft agreed with my concerns, but the lack of knowledge being shared by Microsoft was largely deliberate due to the complexities of tape library sharing mentioned above.  The shortage of troubleshooting advice is a way to prevent any unwanted steps being followed that could make a bad situation, even worse!

I’m sure for the more cynical IT admin, this may come across as being fobbed off, but I do tend to agree with the approach.  All the library sharing support tickets I have raised with MS have all been resolved, and I have never lost any protected data.  Would I be able to say the same if I had been playing in SQL, with DTC, with WMI etc. to fix the problem??? Probably not.  Of course I would like more information published, as I have a need to know how things work, but I feel happier knowing the reasons why there is so little.

The best troubleshooting post Microsoft have for tape library sharing issues is available here, http://blogs.technet.com/b/dpm/archive/2011/12/20/how-to-troubleshoot-tape-library-sharing-configuration-issues-in-dpm-2010.aspx.  It states it is relevant for DPM 2010, but it is equally relevant with DPM 2012 and DPM 2012 R2.  If nothing in this article resolves your issue, you are probably going to need to raise a support ticket with Microsoft.

Happy Data Protecting!

David