Keep can perform efficient copy-on-write of files and directories, but POSIX doesn't provide an API for this. We've decided not to abuse standard hardlinks: while similar (in the "fast copy" sense), hardlinks offer incompatible semantics ("two filenames refer to the same data; writes to either file are reflected in both").

Possible approaches for exposing COW capability through arv-mount:

• Use BTRFS clone ioctl() (requires support for handling ioctl() in llfuse). User can use cp --reflink
• Use s3fs approach of writing a special xattr() to a special place to request a COW link. User uses a custom command to communicate with the file system.

Meanwhile, the following workaround is possible without modifying the FUSE driver (and could be provided as a "copy" CLI program):

• Determine source and target collections, perform the operation using Arvados SDK. Results show up in target directory on refresh.