Windows Vista : Deploying Applications - Choosing a Deployment Strategy

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Most companies share a common goal: create a corporate-standard desktop configuration based on a common image for each operating system version. They want to apply a common image to any desktop in any region at any time, and then customize that image quickly to provide services to users.

In reality, most organizations build and maintain many images—sometimes even hundreds of images. By making technical and support compromises and disciplined hardware purchases, and by using advanced scripting techniques, some organizations have reduced the number of images they maintain to between one and three. These organizations tend to have the sophisticated software distribution infrastructures necessary to deploy applications—often before first use—and keep them updated.

Business requirements usually drive the need to reduce the number of images that an organization maintains. Of course, the primary business requirement is to reduce ownership costs. The following list describes costs associated with building, maintaining, and deploying disk images:

  • Development costs Development costs include creating a well-engineered image to lower future support costs and improve security and reliability. They also include creating a predictable work environment for maximum productivity balanced with flexibility. Higher levels of automation lower development costs.

  • Test costs Test costs include testing time and labor costs for the standard image, the applications that might reside inside it, and those applications applied after deployment. Test costs also include the development time required to stabilize disk images.

  • Storage costs Storage costs include storage of the distribution points, disk images, migration data, and backup images. Storage costs can be significant, depending on the number of disk images, number of computers in each deployment run, and so on.

  • Network costs Network costs include moving disk images to distribution points and to desktops. The disk-imaging technologies that Microsoft provides do not support multicasting, so network costs scale linearly with the number of distribution points you must replicate and the number of computers to which you’re deploying.

As the size of image files increases, costs increase. Large images have more updating, testing, distribution, network, and storage costs associated with them. Even though you only update a small portion of the image, you must distribute the entire file.

Thick Images

Thick images are monolithic images that contain core applications and other files. Part of the image-development process is installing core applications prior to capturing the disk image, as shown in Figure 1. To this date, most organizations that use disk imaging to deploy operating systems are building thick images.

Figure 1. The thick image process.

The advantage of thick images is simplicity. You create a disk image that contains core applications and thus have only a single step to deploy the disk image and core applications to the destination computer. Thick images can also be less costly to develop, as advanced scripting techniques are not often required to build them. In fact, you can build thick images by using BDD with little or no scripting work. Finally, in thick images, core applications are available on first start.

The disadvantages of thick images are maintenance, storage, and network costs. These costs rise with thick images. For example, updating a thick image with a new version of an application requires you to rebuild, retest, and redistribute the image. Thick images require more storage and use more network resources in a short span of time to transfer.

If you choose to build thick images that include applications, you will want to install the applications during the disk-imaging process. I

Thin Images

The key to reducing image count, size, and cost is compromise. The more you put in an image, the less common and bigger it becomes. Big images are less attractive to deploy over a network, more difficult to update regularly, more difficult to test, and more expensive to store. By compromising on what you include in images, you reduce the number you maintain and you reduce their size. Ideally, you build and maintain a single, worldwide image that you customize post-deployment. A key compromise is when you choose to build thin images.

Thin images contain few if any core applications. You install applications separately from the disk image, as shown in Figure 2. Installing the applications separately from the image usually takes more time at the desktop and possibly more total bytes transferred over the network, but spread out over a longer period of time than a single large image transfer. You can mitigate the network transfer by using trickle-down technology that many software distribution infrastructures provide.

Figure 2. The thin image process.

Thin images have many advantages. First, they cost less to build, maintain, and test. Second, network and storage costs associated with the disk image are lower, because the image file is physically smaller. The primary disadvantage of thin images is that post-installation configuration can be more complex to develop initially, but this is offset by the reduction in costs to build successive images. Deploying applications outside of the disk image often requires scripting and usually requires a software distribution infrastructure. Another disadvantage of thin images is that core applications aren’t available on first start, which might be necessary in high-security scenarios.

If you choose to build thin images that do not include applications, you should have a systems-management infrastructure, such as Microsoft Systems Management Server (SMS) or Microsoft System Center Configuration Manager (SCCM), in place to deploy applications. To use a thin image strategy, you will use this infrastructure to deploy applications after installing the thin image. You can also use this infrastructure for other post-installation configuration tasks, such as customizing operating system settings.

Hybrid Images

Hybrid images mix thin and thick image strategies. In a hybrid image, you configure the disk image to install applications on first run, giving the illusion of a thick image but installing the applications from a network source. Hybrid images have most of the advantages of thin images. However, they aren’t as complex to develop and do not require a software distribution infrastructure. They do require longer installation times, however, which can raise initial deployment costs.

An alterative is to build one-off thick images from a thin image. In this case, you build a reference thin image. After the thin image is complete, you add core applications, capture, test, and distribute a thick image. Testing is minimized because creating the thick images from the thin image is essentially the same as a regular deployment. Be wary of applications that are not compatible with the disk-imaging process, however.

If you choose to build hybrid images, you will store applications on the network but include the commands to install them when you deploy the disk image. This is different than installing the applications in the disk image. You are deferring application installs that would normally occur during the disk-imaging process to the image-deployment process. They become a post-installation task. Also, if you have a systems-management infrastructure in place, you will likely use it to install supplemental applications post-deployment.

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