Reserved storage, shadow copies, WinSxS: what the words mean
The vocabulary Windows uses for storage, defined plainly, with what each term means for the space on your drive.
Storage on Windows is described with words that sound technical and are rarely defined. Each has a plain meaning, and knowing them turns most storage confusion into an ordinary problem.
The terms
| Term | What it actually means |
|---|---|
| Reserved storage | Space Windows holds back so updates always have room |
| Shadow copy | A snapshot of a volume, used by System Restore and backup |
| Restore point | One shadow copy, plus registry state |
| WinSxS | The component store, built from hard links, not as big as it looks |
| Hard link | A second name for the same file. Counted twice by naive tools |
| Junction | A folder that points at another folder |
| Sparse file | A file that reports a size larger than it occupies |
| Size on disk | What a file occupies including cluster rounding and compression |
| System Volume Information | Per volume folder holding restore and index data |
| Page file | Virtual memory, on disk |
| Compact OS | Windows stored compressed, to fit small drives |
The three that cause most confusion
Hard links explain why WinSxS looks enormous: the same bytes counted under several names. Sparse files explain why a WSL disk reports 60 GB and occupies 12. Shadow copies explain why deleting files does not free space immediately, because a snapshot still references the old data.
Size against size on disk
Right click a file, Properties, and Windows shows two numbers. Size is the contents. Size on disk counts whole clusters, so thousands of tiny files occupy more than their sizes add up to, and a compressed file occupies less. When freeing space, size on disk is the one that matters.
Why tools disagree
- Explorer counts what it can walk and understands some links.
- A folder walk counts every name, so it over-reports hard linked stores.
dirand PowerShell report the same way unless told otherwise.- Windows' own storage screen sorts into categories rather than folders.
- Disk Management reports partitions, which is a fifth answer.
None of them is wrong. They answer different questions, and differences of several gigabytes between them are normal on a healthy machine.
Common questions
What is reserved storage in Windows?
Space Windows sets aside so feature updates always have room to install. It is a few gigabytes and can be turned off with DISM, at the cost of updates failing when the drive is full.
What is a shadow copy?
A snapshot of a volume at a moment in time, used by System Restore and backup software. Deleted files stay on disk while a shadow copy still references them, which is why space does not always free immediately.
Why does WinSxS look so big?
Because it is built from hard links: the same bytes appear under several names and naive tools count them once per name. DISM reports the actual size, which is far smaller.
What is the difference between size and size on disk?
Size is the contents of the file. Size on disk counts whole clusters and accounts for compression, so it is the number that matters when freeing space.