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You encountered a .zst file from a package manager, database backup, or modern data pipeline. Zstandard (zstd) is Facebook/Meta's answer to the question "what if gzip were fast?" — it matches or beats gzip's compression ratio while being 3-5x faster at decompression.
Developed by Yann Collet (who also created LZ4), Zstandard was designed for real-time compression at scale. It supports adjustable compression levels from 1 (fast, light) to 22 (slow, tight), and its dictionary compression mode is particularly effective for small files that share common patterns — which is why databases and log aggregation systems love it. The format is open-source and increasingly showing up in Linux package managers (Arch, Fedora), kernel compression, and data engineering tools.
7-Zip 23+ opens .zst files on Windows. The `zstd` command-line tool (available via package managers on Linux/macOS) handles compression and decompression. On macOS, `brew install zstd`. Zstandard is the newest format in this category and the fastest-growing — it's what gzip would be if it were invented today.