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Booting Windows from a compressed image

Dream Start

Article from ADMIN 25/2015
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Microsoft's WIMBoot lets you run Windows 8.1 from a compressed file on solid state drives.

Recent Windows systems use Windows Image file format (WIM) as a disk image format. Unlike sector-based equivalents such as ISO, which require special handling by the system, WIM is a file-based format. System files and other important information are stored in compressed form within the WIM image.

Windows 8.1 rolls out a new feature called Windows image file boot (WIMBoot). According to Microsoft, WIMBoot "… lets you set up a PC so that it runs directly from a compressed Windows image file. WIMBoot can significantly reduce the amount of space used by Windows files" [1].

WIMBoot is designed to reduce the required space for a new generation of Windows tablets, but it will also work on other Windows 8.1 systems if you're looking for a way to free up disk space.

Instead of placing the system files directly on the Windows partition, WIMBoot uses pointers that reference files on the WIMBoot image. For this reason, WIMBoot only makes sense for solid state drives. According to Microsoft, "WIMBoot isn't supported on traditional drives that contain rotational components or on hybrid drives that contain both solid-state and rotational drive components. WIMBoot works by taking advantage of the capability of solid-state drives to access different areas of the hard drive quickly" [1].

WIMBoot only requires 4GB of disk space in Windows 8.1. WIMBoot does not impose restrictions on users. Programs and tools are installed as usual. Rolling out updates and patches for Windows 8.1 with the WIM file is generally the same as booting from a Virtual Hard Disk (VHD) file, only faster. You can even create your own WIM image on the basis of a preinstalled Windows 8.1 computer.

In this article, I will describe how to perform the necessary preparations for using WIMBoot on a Windows 8.1 system, how to install the technology, and your options when creating the image.

Prerequisites for WIMBoot

The new Windows 8.1 Update 1 allows you to roll out an entire operating system via a single WIM file and keep it up to date. Deploying Windows 8.1 via WIMBoot is fast and uses less memory, so the technology is suitable for speeding up the distribution of Windows 8.1 throughout an enterprise.

To use WIMBoot in your company, you will need the latest Windows 8.1 Update 1 of the Windows Assessment and Deployment Kit (ADK) [2]. WIMBoot is mainly implemented using the dism.exe tool and the Windows Preinstallation Environment (WinPE) version 5.1 from the ADK. WinPE version 5.0 in the Windows 8.1 ADK does not yet support this technology. The tools QuickPE [3] and Rufus [4] can also help create a suitable WinPE image and bundle it into a bootable USB stick. To implement the procedures in this article, you can use the trial version of Windows 8.1 Enterprise [5].

How WIMBoot Works

If you install Windows 8.1 in the usual way, the necessary files from the install.wim file in the sources directory of the installation DVD are extracted and written to the computer hard disk. Additionally, important system files land in compressed form on the hard disk for recovery routines (Reset and Refresh) to fix a system, if needed. Not surprisingly, these memory operations require a lot of disk space. However, if WIMBoot is used, Update 1 uses the WIM image for recovery, thus saving a huge amount of disk space. Furthermore, only this one file needs to be installed on computers. By booting this file, Windows 8.1 starts just as it does in a normal installation.

In addition to booting the default install.wim file, you can create another WIM file (custom.wim) and connect it to the computer. The default Update 1 files are located in install.wim, whereas custom.wim contains changes, patches, installed programs, and other settings that differ from the default files.

Another option is to create a WIM file based on an already existing installation and then assign this WIM file to a system. The PC then boots with the same settings and data as before but uses less disk space.

The Reset and Refresh functions also work with WIMBoot. You only need to connect a third WIM file (winre.wim), through which the recovery is performed. By separating this file from the default install.wim file, you thus save space. The recovery file is saved as a hidden file in the install.wim image in the path Windows\ System32\Recovery\winre.wim.

Only from SSD with UEFI

Windows 8.1 is not capable of booting from WIM files without Update 1. This also applies to the tool dism.exe. Windows Server 2012 R2 does not support this function after installing Update 1, so you can only boot workstations via WIMBoot with Windows 8.1.

Moreover, use of this technology is only possible on new PCs with UEFI. WIMBoot might not support the BIOS, but it does support the UEFI system's secure boot technology. In a Windows 8.1-based Hyper-V environment, you must accordingly use second-generation VMs that support UEFI.

Although the system cannot boot from conventional or hybrid drives, you can, of course, connect other drives to the PC in addition to your SSD. You should make sure the virus scanner you use is compatible, as well, because not all scanners support WIMBoot.

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