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Differencing Images: Making the Difference!



One feature that I have started to utilize in SmartDeploy Enterprise is differencing images. This technology is currently available for you to reduce your image development time, decrease the amount of space required for OS image storage, and improve the change management process of 'certifying' OS images in your IT organization. A simple way to think about differencing images is that they represent only the changes (delta) between image ‘A’ and image ‘B.’ Most IT shops have at least a dozen different images they are supporting and maintaining which hogs valuable network space and more importantly takes up unnecessary time in capturing or deploying large image files during development.

For example, if I want a Windows 7 image for each of my departments (HR, Finance, and IT) then I need 3 different images right? Yes and no. No, we do not need 3 huge 10GB images containing all the Windows base OS files and customized software for each of our three departments. First, we can capture a standard base image containing our basic applications (anti-virus software, web plug-ins, and Office). Then we can capture separate ‘differencing’ or delta (the changes) images between our base image and our customized software and settings for each of our three departments.

Follow along: I captured a base image, aka a standard .WIM file of Windows 7 which ended up being about 5.8GB. Now I want to customize my image file to be specific for IT Professionals. So, I booted my VMware Workstation file of Windows 7 which I had recently captured. Then I installed two programs that all of my IT Professionals need, the Windows Automated Installation Kit (WAIK) and VMware Workstation 7. Next, I shut down the virtual machine and run the Capture Wizard again. This time I point to my original .WIM file (from my first capture run of the base image file) and create a new .DWM file with a similar name.

Now I have two image files: Win7x86_GI_2-1-10.wim and Win7x86_GI_IT_diff_2-1-10.dwm that are saved in the same folder/directory. This aspect is important because when you deploy a .DWM image file you need to have the master image file residing in the same directory for the image deployment to complete successfully.

My new .DWM or differencing file is only 3GB which saves about two-thirds of the space required to store the entire IT Department image. My base image was almost 6GB. If I had created a new traditional WIM, I would have had a new 9GB image for my IT Department. In total, the traditional method would require 15GB, with differencing images it only requires 9GB. This process can be continued to create .DWM files for each variation of your base image. To do this easily, you could have 3 separate virtual machines of the same base Windows 7 OS. Then customize each virtual machine with the different applications and settings for your 3 departments. Typically, I save these virtual machines to revisit at a later date to re-capture after more recent patches and security updates have been released for various applications.

From a change management perspective, differencing images allow you to control the image certification process. You can distribute a 'certified' master image that gets a rubber stamp of approval every couple of months. Then encourage IT support staff to focus on developing differencing images while they await the quarterly or semi-annual master image.

Overall, I like using differencing images with SmartDeploy Enterprise and I think you will as well. Please see me next post for a detailed screen shot based approach on how to 'difference' image step-by-step and for some .DWM file deployment best practices.

Aaron Lines

IT Consultant

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Can you capture app-level differentials, and apply mulitple DWMs?  We have a need to deploy a variation of 10 different applications, random combinations of them, to laptops of different hardware.  I need to deploy between 2 and 6 applications per laptop and need to be able to choose from a number of apps.  Any way to do that?


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