Microsoft once claimed the original 64 GB Surface Pro would have only 23 GB of usable free space — that’s more than half used for system files! But Windows now fits on 16 GB drives.

These 16 GB devices even have room to spare. As part of Windows 8.1 Update, Microsoft rolled out a new feature that allows Windows to fit on drives with a very small amount of storage space.

Why Windows 8 Needed So Much Space

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Older Windows 8 devices — like the Surface Pro — seemed to gobble space like crazy. While Microsoft originally announced the original 64 GB Surface Pro would have only 23 GB available, the device actually shipped with 30 GB available. Still, that’s a huge amount of storage space used for system files — more than half!

When you or the computer manufacturer install Windows, Windows extracts gigabytes of system files to the system partition. It also creates a recovery partition that can be used to reinstall Windows using the Refresh or Reset features — that uses quite a few gigabytes, too. The WinSXS folder also grows as you install Windows updates, keeping copies of the old files Windows Update replaced. Microsoft has struggled to make Windows use less space.

Windows Image File Boot, aka WIMBoot

Windows 8.1 Update introduced a new feature called “Windows Image File Boot,” also known as “WIMBoot.” Rather than using the traditional method of extracting Windows’ system files from an image file and placing them on the system partition, a Windows system installed with WIMBoot keeps the compressed .wim image files around. These .wim files are stored on a separate “images” partition, just like the Windows recovery image is stored on a separate partition on a typical Windows system.

DISM (the Deployment Image Servicing and Management) tool creates “pointer” files on the standard Windows system partition, and these pointer files point to files inside the compressed .wim images. The computer boots normally and your typical C: system drive looks just like it normally would.

However, in the background, those typical Windows system files aren’t actually stored on your system partition. They’re compressed on a .wim file on another partition, and Windows transparently loads them from the .wim file and decompresses them whenever it needs them. This saves a huge amount of space because the files can stay compressed. Here’s an image from Microsoft’s blog post on the subject that shows how the typical partitioning scheme looks:

Isn’t This Slower?

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There’s obviously more overhead when the system has to decompress files from a compressed image before opening them. It’s a bit like using NTFS compression — it’s not a great idea to use in most cases, as it will often just slow things down. WIMBoot will typically be slower than a standard Windows install. You shouldn’t use BitLocker with WIMBoot, and Microsoft even says some antivirus and backup tools may be incompatible with it.

WIMBoot can only function on solid-state drives (SSDs) and similar eMMC drives. It can’t be used on rotational drives or hybrid drives. As Microsoft puts it, “WIMBoot works by taking advantage of the capability of solid-state drives to access different areas of the hard drive quickly.”

In some specific cases, WIMBoot might be even faster. Picture a very slow eMMC drive that reads files slowly in combination with a fast CPU that can decompress files quickly. It’s possible that WIMBoot would be faster — the eMMC drive can read the smaller compressed data and the CPU can decompress it faster than the slow eMMC drive could read a larger amount of uncompressed data. However, on systems with good solid-state drives with fast performance, WIMBoot will be slower.

How Much Space Does WIMBoot Need?

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Here’s the biggest news yet: With WIMBoot, Windows can be installed to just 4 GB of space or so. In other words, manufacturers can make 16 GB Windows tablets or laptops and 12 GB of their space will be available for applications and user data. This is huge, and it lets Windows compete in the same space as cheap Android tablets and Chromebooks. Windows doesn’t need a much larger drive just to offer the same amount of free storage space to users.

Combined with the free Windows 8.1 with Bing operating system, computer manufacturers can now offer much cheaper PCs — we may see the return of netbooks.

How To Get WIMBoot

WIMBoot is a feature intended for computer manufacturers who can install Windows with WIMBoot to save space on devices with small amounts of storage — 16 GB or 32 GB, typically. You’ll get a Windows system installed with WIMBoot when you buy one of these new “Windows 8.1 Update” PCs with a small amount of included storage.

Microsoft does offer a guide to creating WIMBoot images, but it’s not intended for the average Windows geek. Besides, if you already have a Windows PC — even one with a small 64 GB of storage — you’re probably better off not using WIMBoot. Using WIMBoot will just slow down your PC, even if you go through the trouble of setting it up properly. Sure, you could theoretically get some additional space — but it probably isn’t worth the cost.

The next time you see a 16 GB Windows device, don’t laugh — it might have been too small to fit a user’s file-s and applications in the past, but Windows now fits on such a drive with room to spare.

Image Credit: Chris F on Flickr, Simon Wullhorst on Flickr