Compression
Encoding data to take up less space, often transparently at the filesystem level.
ZFS and Btrfs offer transparent compression (like lz4/zstd) that can actually improve performance while saving space, because less data is read from and written to disk.
Related articles
ZFS
An advanced filesystem and volume manager offering data integrity, snapshots, compression and software RAID.
Btrfs
A modern Linux filesystem with snapshots, checksums and built-in RAID-like features.
Deduplication
Storing only one copy of identical data blocks to save space, especially in backups.