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Btrfs (alternatively: butterfs/betterfs) is a native Linux filesystem and set of maintenance utilities developed to deliver similar functionality to ZFS. It is a new copy on write (CoW) filesystem aimed at implementing advanced features while focusing on fault tolerance, repair and easy administration. While the format itself is now marked as stable, the entire filesystem is not yet ready for production use, and you shouldn't trust your data to it. However, it is in active development and being improved with each release.