That's for a journalling filesystem. For CoW filesystems, the issue is that any change to the filesystem is done by making a new file tree containing your chang

That's for a journalling filesystem. For CoW filesystems, the issue is that any change to the filesystem is done by making a new file tree containing your change, and then updating the root to point the new tree. Later, garbage collection finds files that are no longer part of an active tree and returns their storage to the pool. Changes are usually batched to reduce the amount of tree changes to a manageable amount. A bonus of this design is that a filesystem snapshot is just another reference to a particular tree. This requires space, but CoW filesystems also usually reserve an amount of emergency storage for this reason.