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From: Robert Ayrapetyan <robert.ayrapetyan@gmail.com>
Date: Sun, 21 Apr 2024 10:37:07 -0700
Message-ID: <CAAboi9s9=h-ULoTJ4kcTi3S297RWou0JfBz5nTQP90pVpA37bA@mail.gmail.com>
Subject: NFS server side folder copy
To: linux-nfs@vger.kernel.org
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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.