In a blog post, Microsoft explains that the upcoming large Windows 10 1903 update will reserve disk space to be used by updates, apps, temporary files, and system caches. Initially Microsoft will start with about 7 GB of reserved storage that can't be deleted and used for anything else. Reserved storage will be introduced on clean installs and preinstalled versions of Windows 10 1903.
Microsoft argues that reserved storage will ensure that critical OS functions always have access to disk space. The company also explains that, “Windows and application scenarios may not work as expected if they need free space to function.”
Reserved storage works by automatically storing temporary files from apps and system process into the reserved disk space. Windows will not report that the temporary files consume free disk space as long as the reserve isn't full. It also makes it easier for Windows to remove unneeded temporary files through Storage Sense. If, for some reason the reserved storage fills up, Windows will continue to work, and will temporary consume some disk space outside the reserve.
When Windows 10 needs an update, it can remove the temporary unneeded OS files from the reserved storage, so the update can use the full reserved storage capacity. This should ensure that most PCs are able to download and install a Windows update, even when they appear to have minimal free disk space. If the update needs more space than available in reserved storage, it will also temporary use regular disk space. If that even isn't sufficient space to apply the update, Windows will start a wizard to help you free up space or to help you to temporarily extend your disk pace with external storage such as USB drive or external HDD.
With Windows 10 Update 1903, Microsoft expects it will need a reserved storage space of 7 GB which can increase, or decrease, based on system usage.
Users will not be able to remove the reserved storage space, but will be able to limit the disk space it uses. Normally, Microsoft should release Windows 10 Update 1903 in March/April this year as the Windows 10 April 2019 Update.
