How does bitrot protection actually work?

[quote="http://en.wikipedia.org/wiki/Copy-on-write":1cqyxbmf]Copy-on-write (sometimes referred to as "COW") is an optimization strategy used in computer programming. Copy-on-write stems from the understanding that when multiple separate tasks use initially identical copies of some information (i.e., data stored in computer memory or disk storage), treating it as local data that they may occasionally need to modify, then it is not necessary to immediately create separate copies of that information for each task. Instead they can all be given pointers to the same resource, with the provision that on the first occasion where they need to modify the data, they must first create a local copy on which to perform the modification (the original resource remains unchanged). [/quote:1cqyxbmf]
This is the core idea. When snapshots are taken, the snapshot folder and the main folder both initially have pointers to the same data on the disk. The btrfs wiki calls this "cloning" to distinguish it from linux hard links.

When the file is modified, then the file is fragmented, so the unchanged blocks remain referenced by both folders. For the blocks that have been changed, the original block ends up referenced only by the snapshot, and the changed block is referenced by the main folder.

When you have multiple snapshots, the idea is simply extended to cover them all.

CoW is not limited to snapshots, there is a --reflink option in the cp command which has the same properties. Initially the two copies share the same datablocks, but as the files are modified only the shared blocks remain in common - resulting in fragmentation, but efficient use of disk space.

I'm not sure why Netgear linked bit-rot protection to CoW, it is an odd admixture. From what little has been posted here, bit-rot protection depends on btrfs file checksums and RAID, not CoW.

The obvious use of CoW is to create snapshots, which is a space-efficient mechanism that allows you to roll back to previous versions of the files. If you have large files with a few differences between them, then CoW could be used (e.g. cp --reflink) to reduce disk space. If you have a folder structure that contains source code, CoW is one way to create a development branch - again one that is space efficient. It isn't well suited to files that are being continuously updated (for instance torrent files being downloaded, or databases that are always changing). Snapshots and cp --reflink are very fast operations; the performance hit happens later on when the files are modified.