Cottleston Pie

What I really need at work sometimes is a filesystem where I can create copy-on-write links. The motivation in this particular instance was a large job where I needed to copy a directory of almost ten thousand files from another user's run because when I invoked a long run of my own, the program might want to tweak one or two of the files in a minor way. It probably wouldn't need to touch any of them. But if it did, the file had to be such that it could write the modification.

So, I'm thinking the filesystem should be smart about it, like memory, and not actually copy the file data but just create new directory records that reference the original records. This would be similar to a hard link and would have the same limitations. The difference would be that the file would have its own permissions (meta-data) and the first change (write) to the file's data content would cause the data to be copied prior to modification.

Operating systems have been doing this with memory for years, so this isn't a new idea, just a different context.