Inside a WIM File

Aside from storing all the file data, it also keeps the file metadata and ACLs.  This means it stores reparse points and alternate data streams.  WIM has very good compression and it stores each file only once, to save space on duplicate files and hard links (which appear in Windows images quite a bit.)

In short, you can back up your entire operating system to a WIM file and it stays intact.  It’s great for backups.  And it’s great for deploying images.

The other great part of WIM is that it’s very modular.  You can separate WIMs into pieces and you can reference other WIM files, if you want.

In fact, every Vista install disc has an install.wim on it that contains images of each version of Vista represented on the disc (Vista Home, Vista Business, etc.)  It’s the efficiency of the WIM format that allows all of these different versions to fit on a single disc.

One thing that isn’t often discussed is the various types of WIM formats.

• WIM: A normal WIM file contains several metadata sections (one for each image) and a big file data section.
• SWM: A split WIM file stores several metadata sections in the first file, then splits the file data into several files.  This is designed for spanned media.
• RWM: A resource WIM file stores just the file data.  This type of file isn’t documented and can’t be created with ImageX, though it can be applied with ImageX.  (Even normal WIMs and split WIMs can be used as resource WIMs.)
• MWM: A metadata WIM file stores just the metadata.  Like resource WIMs, you can’t create these with ImageX.  Also, we tend to use the .wim extension with these sorts of files, because this file has everything needed to recreate an image, it’s just missing the file backing data.

The flexibility of the WIM format lies in being able to separate the file data from the metadata.  All of the above file formats are different combinations of the two.

And why is that?  What good is separating the metadata and the file data?

Consider the following:

1. The metadata for an image is usually just a few megs.  But the file data can be several gigs.
2. One metadata file can refer to several resource files. (Using the /REF flag in ImageX.)
3. When applying an image, you can refer to one metadata file and whatever resource files you want.  As long as they collectively contain all the file data, it doesn’t matter what they’re named

With this in mind, you can create a DVD with file data for a bunch of images.  Then, later, you can distribute new metadata for download over the Internet, if you have an update.

Or, you could create images that use resources from Vista’s install disc, if you know that your users will already have that disc.

WIM goes a bit beyond what other compression formats do by giving you the chance to split up the files as you choose and even gives you the option to rely upon previously created files.