Image group in Windows Deployment
An image group is a collection of .wim files that share common file resources and security. Servicing an image within an image group (such as applying a hotfix or a service pack or updating files) requires exclusive access to the entire image group. File resources are shared across the image group (single-instanced) even though the metadata of each image resides in a separate physical .wim file. Image groups contain two file types:
l Res.rwm. Contains the file streams for images as defined in Install.wim, Install2.wim, and WinXP.wim. Note that each image group has its own Res.rwm file.
l Install.wim. Contains image metadata that describes the content of an operating system image. The actual file resources for the image reside in Res.rwm.
Each image group will have a Res.rwm file created when the first image is added to the image group. All resources for all files reside in Res.rwm. The Res.rwm file is a .wim file that is renamed to differentiate the resource-only .wim file from the metadata .wim files and to speed up image enumeration. Because image enumeration only works on .wim files, the Res.rwm file will be skipped.
The .wim file format uses single-instancing technology, so the disk storage requirements for images within an image group are significantly reduced.