Have a CoW, man

Even if you have no redundancy in your next-gen filesystem, it will be significantly more robust than its forbears. This is thanks to a technique called Copy-on-Write (CoW): a new version of a file, instead of overwriting the old one in-place, is written to a different location on the disk. When, and only when, that is done, the file’s metadata is updated to point to the new location, freeing the previously occupied space. This means that if the system crashes or power fails during the write process, instead of a corrupted file, you at least still have a good copy of the old one Besides increased reliability, CoW allows for a filesystem (or more precisely a subvolume) to be easily snapshotted. Snapshots are a feature, or even the feature, that characterises our nextgeneration filesystems. A snapshot behaves like a byte-for-byte copy of a subvolume at a given time (for now think of a subvolume as a glorified directory – the proper definition is different for btrfs and ZFS), but when it is initially taken, it takes up virtually no space. In the beginning, the snapshot just refers to the original subvolume. As data on the original subvolume changes, we need to preserve it in our snapshot, but thanks to CoW, the original data is still lying around; the snapshot is just referred to the old data, so the filesystem will not mark those blocks as unused, and old and new can live side by side. This makes it feasible to keep daily snapshots of your whole filesystem, assuming most of its contents don’t change too drastically. It is even possible to replicate snapshots to remote pools via SSH.