7zip encrypted 7z file seems to double count it’s storage, why?

Understanding Storage Behavior When Using Encrypted 7zip Archives: A Case Study

Managing large multimedia collections often necessitates secure and efficient storage solutions. Among popular tools, 7-Zip stands out for its robust encryption and compression capabilities, making it a preferred choice for users looking to safeguard their data. However, users may encounter unexpected storage behavior when working with large encrypted archives. This article explores a common scenario, analyzing the causes behind apparent storage doubling during archive updates and offering best practices for proper file management.

Case Overview

Consider a user who has stored approximately 70GB of home videos within a password-protected and encrypted 7-Zip archive. After deleting the original videos from their system—leaving only the archive—the total storage consumption reports about 450GB of used space on a 500GB drive.

The user then attempts to add a 500MB video file to the existing archive. During this operation, 7-Zip initiates an update process, seemingly requiring around 70GB of free space, and ultimately fails with a disk space error. The user suspects that the updating process temporarily demands significantly more storage, leading to confusion over why storage appears to “double.”

Analyzing the Issue

Understanding 7-Zip’s Update Process:
When adding files to an encrypted archive, 7-Zip typically creates a new archive by compressing the existing data plus the new files. The original archive isn’t overwritten directly; instead, a new archive is constructed, and the old one is replaced. This process involves temporary storage to hold the intermediate data, which can temporarily double the needed disk space.
Temporary Storage Requirements:
During the update, 7-Zip may require additional space proportional to the size of the archive and the files being added—to hold the new archive momentarily before replacing the old one. If sufficient free space isn’t available at this staging phase, operations may fail or seem to “double” storage usage.
Why the Increase in Storage?:
Compression Process: Creating the new archive requires reading existing data, compressing it, and writing it to a new file.
Encryption Overheads: Encrypting file names and data adds processing and temporary buffer requirements.
Deletion of Original Data: Removing the original videos reduces total used space but doesn’t immediately free space until the new archive is finalized.