Provide a way to create clone/CoW copies

[candrews]

Currently, copying files is implemented by reading the data of the source file and writing it to the new file, see fs-extra's implementation of copy. The problem is that this approach doesn't allow the OS to optimize file copying (such as by using direct disk-to-disk copy if it's available), nor does it allow the (optional or by default) use of reflink/CoW.

I'm specifically interested in making CoW (which is already available on some *nix file systems, including btrfs and zfs) available to nodejs.

IMHO, nodejs should add an fs.copy function (just like what fs-extra already does) which is implemented using the OS native file copying syscall. It should have an optional option parameter that allows specifying the copy should use CoW (if available), just like how cp --reflink=auto from coreutils works on Linux.

Note that this would be particularly benefit for cases where files are copied often - particularly big files. I'm thinking that this improvement would substantially benefit package managers (npm, yarn, etc).

[eljefedelrodeodeljefe]

Likely needs libuv implementation. Related: libuv/libuv#925

[Daniel15]

Also see some of the attempts at using native copy calls in Yarn:

yarnpkg/yarn#3290 which uses https://github.com/sciolist/fcopy (Unix via a native module)
yarnpkg/yarn#2960 (Win32 via node-ffi)

[richardlau]

fs.copyFile was added by #15034.

[cjihrig]

FWIW, CoW is not supported yet. There is a PR to add it to libuv but I haven't landed it yet because I have yet to see it work on any of the CI machines.

[richardlau]

Let's keep this open then.

[coderaiser]

Ponyfill fs-copy-file can be used for copying files. In node.js < v8.5.0 it copies files with help of streams and for v8.5.0 and next versions it uses native implementation of fs.copyFile.

[Daniel15]

@coderaiser - That looks like a useful module. Although, technically that's not a ponyfill as ponyfills never use the native API (as per the document you linked to) 😃

[cjihrig]

Forgot to link it, but this is closed in a16d88d.