With the Windows 8.1 Update, Microsoft shrank the Windows 8.1 install footprint to make it suitable for low-cost tablets with just 16GB of permanent storage, a reduction from the 32GB generally required for Windows 8. Windows 10 will shrink the disk footprint further, potentially freeing as much as 6.6GB of space on OEM preinstalls.
Microsoft describes two sources of savings. The first is the re-use of a time-honored technique that fell out of fashion as hard drives grew larger and larger: per-file compression.
The NTFS filesystem used in Windows has long allowed individual files and folders to be compressed, reducing their on-disk size at the expense of a small processor overhead when reading them.
With spinning disks getting so large as to feel almost unlimited, per-file compression felt like a relic from a bygone age by the mid-2000s. But with the rise of solid state storage and ultra-cheap devices with just a handful of gigabytes available, per-file compression has gained a new lease on life.
When installing Windows 10 from scratch, it will assess the system’s performance to figure out if the system processor is fast enough that it can decompress system files without any noticeable performance impact. If it’s fast enough (and it’s hard to imagine a system built in the last decade that wouldn’t be fast enough, though Microsoft doesn’t appear to have disclosed the exact requirements) then a selection of system files will be stored compressed on disk.
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