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From: Benjamin Coddington <bcodding@redhat.com>
To: Robert Ayrapetyan <robert.ayrapetyan@gmail.com>
Cc: linux-nfs@vger.kernel.org
Subject: Re: NFS server side folder copy
Date: Tue, 23 Apr 2024 07:00:15 -0400
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On 21 Apr 2024, at 13:37, Robert Ayrapetyan wrote:

> 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).

Hi Robert,

Looks like `cp` is serialized?  Is there a way to send COPY/CLONE in
parallel?

> 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.

The NFSv4 protocol doesn't have an operation that can do a server-side
copy/clone of a folder and all its files.  It'd be quite an interesting one,
though.  The failure and recovery cases would be sprawling.

Ben