? ZFS is copy-on-write.
> With APFS, if you copy a file within the same file system (or possibly the same container; more on this later), no data is actually duplicated. Instead, a constant amount of metadata is updated and the on-disk data is shared. Changes to either copy cause new space to be allocated (this is called "copy on write," or COW). btrfs also supports this and calls the feature "reflinks"—link by reference.
> I haven't seen this offered in other file systems (btrfs excepted), and it clearly makes for a good demo, but it got me wondering about the use case. Copying files between devices (e.g. to a USB stick for sharing) still takes time proportional to the amount of data copied of course. Why would I want to copy a file locally? The common case I could think of is the layman's version control: "thesis," "thesis-backup," "thesis-old," "thesis-saving because I'm making edits while drunk."
CoW is one of those features that is superficially questionable until you start noticing the bits and pieces of workflow it really makes faster and easier. Given the keynote was towards an audience of developers, I'm really surprised that there wasn't a demo showing how much faster deploys, etc are with such tech.