Microsoft Explains OS Compression in Windows 10

Posted on March 16, 2015 by Paul Thurrott in Windows 10 with 0 Comments

One of the many things that makes Windows 10 unique is that this version of Windows takes up less space on a disk and requires fewer resources than its predecessors. Microsoft accomplishes this magic with a technology it calls OS compression, reducing the storage footprint of Windows 10 by 5.5 GB to 14.6 GB overall.

“With these new and enhanced functionalities, devices running Windows 10 will have more free space for photos, videos, and music,” the Windows team notes in a post to Blogging Windows. “Windows devices can be lightweight and highly mobile, yet, when you need it, have the full capabilities of the Windows OS.”

OS compression is, of course, what makes new low-end devices like the WinBook W700 tablet—which has just 16 GB of onboard flash storage—possible. Apparently squeezing Windows into such a small space—and leaving enough room to at least use the device, if not install a few apps or download some videos—requires a bit of work.

It goes something like this: Windows 10 uses “an efficient compression algorithm” to compress system files and it includes recovery enhancements that have removed the requirement for a separate recovery image.

That system file compression bit gives back 1.5 GB (for 32-bit Windows) or 2.6 GB (for 64-bit) of storage, Microsoft says. And the Push Button Reset functionality—PC Reset and PC Refresh—has been redesigned in this version to not require a separate recovery image. That saves an additional 4 GB to 12 GB of space. (This latter advance applies only to PCs, laptops and tablets, not phones.)

Of course, this raises an obvious question. If there’s no recovery image, how does Push Button Reset even work?

According to Microsoft, “the Refresh and Reset functionalities will [now] rebuild the operating system in place using runtime system files … this take up less disk space [and] it means you will not have a lengthy list of operating system updates to reinstall after recovering your device.” That’s right, it’s a two-fer.

(If things really go south, you will need to have created recovery media, which I’ll be documenting here on this site and in the forthcoming book Windows 10 Field Guide.)

You may know that this stuff isn’t technically new to Windows 10, as it did debut in Windows 8.1 in a less capable form called WIMBOOT. (It has of course been improved in Windows 10.) But this raises an interesting separate issue, too: If you try to upgrade a low-end WIMBOOT-based Windows 8.1-based device with a compressed OS (like the excellent HP Stream 7—just $99 at to the Windows 10 Technical Preview, it fails. But that should change soon.

“Windows 8.1 devices using WIMBOOT are not yet able to upgrade to Windows 10 because many of the WIMBOOT devices have very limited system storage,” Microsoft explains. “That presents a challenge when we need to have the Windows 8.1 OS, the downloaded install image, and the Windows 10 OS available during the upgrade process. We do this because we need to be able to restore the machine back to Windows 8.1 if anything unexpected happens during the upgrade, such as power loss. In sum, WIMBOOT devices present a capacity challenge to the upgrade process and we are evaluating a couple of options for a safe and reliable upgrade path for those devices.”

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