Hello!
Attempting to understand the implementation of server-side copying of
folders mounted via NFS 4.2.
Currently copying an entire folder (cp -r --reflink=always) produces:
newfstatat(AT_FDCWD, "/mnt/source_file", {st_mode=S_IFREG|0644,
st_size=2466, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(3, "dest_file", {st_mode=S_IFREG|0644, st_size=2466, ...}, 0) = 0
openat(AT_FDCWD, "/mnt/source_file", O_RDONLY|O_NOFOLLOW) = 4
newfstatat(4, "", {st_mode=S_IFREG|0644, st_size=2466, ...}, AT_EMPTY_PATH) = 0
openat(3, "dest_file", O_WRONLY|O_TRUNC) = 5
ioctl(5, BTRFS_IOC_CLONE or FICLONE, 4) = 0
close(5) = 0
close(4)
sequence of operations for each file within the directory on the
client side. Notably, the actual file copy occurs on the server side
and is instantaneous (BTRFS_IOC_CLONE).
But for TTLs around 50 ms (such as within intra-regional connections
like US West/East), copying a 700MB directory containing 500 files
takes about 4 minutes (while "true" server-side folder copy is almost
instant).
Exploring if there exists a Linux mechanism enabling the copying of an
entire folder without individually accessing each file within the
directory for server-side copy operation.
