FsObjCopyOnWrite
Implemented in:
The copy-on-write (COW) directory object wraps two directories: a read layer and a write layer. Writes go to the write layer, while reads can come from either layer with the write layer being checked first.
It is slightly misnamed because the actual copy-on-write part is not implemented: if a file is present in the read layer, opening it for writing is rejected. A complete implementation would copy the file and hand back a file descriptor to the copy.