Re: RFC: Does ZFS block cloning do this?

On Wed, Aug 6, 2025 at 8:32 AM Alan Somers <asomers@freebsd.org> wrote:
>
> On Wed, Aug 6, 2025 at 9:18 AM Rick Macklem <rick.macklem@gmail.com> wrote:
>>
>> Hi,
>>
>> NFSv4.2 has a CLONE operation. It is described as doing:
>>    The CLONE operation is used to clone file content from a source file
>>    specified by the SAVED_FH value into a destination file specified by
>>    CURRENT_FH without actually copying the data, e.g., by using a
>>    copy-on-write mechanism.
>> (It takes arguments for 2 files, with byte offsets and a length.)
>> The offsets must be aligned to a value returned by the NFSv4.2 server.
>> 12.2.1.  Attribute 77: clone_blksize
>>
>>    The clone_blksize attribute indicates the granularity of a CLONE
>>    operation.
>>
>> Does ZFS block cloning do this?
>>
>> I am asking now, because although it might be too late,
>> if the answer is "yes", I'd like to get VOP calls into 15.0
>> for it. (Hopefully with the VOP calls in place, the rest could
>> go in sometime later, when I find the time to do it.)
>>
>> Thanks in advance for any comments, rick
>
>
> Yes, it does that right now, if the feature@block_cloning pool attribute is enabled.  It works with VOP_COPY_FILE_RANGE.  Does NFS really need a new VOP?
Either a new VOP or maybe a new flag argument for VOP_COPY_FILE_RANGE().
Linux defined a flag argument for their copy_file_range(), but they have never
defined any flags. Of course, that doesn't mean there cannot be a
"kernel internal"
flag.

So maybe adding a new VOP can be avoided. That would be nice, given the timing
of the 15.0 release and other churn going on.

The difference for NFSv4.2 is that CLONE cannot return with partial completion.
(It assumes that a CLONE of any size will complete quickly enough for an RPC.
Although there is no fixed limit, most assume an RPC reply should happen in
1-2sec at most. For COPY, the server can return with only part of the
copy done.)
It also includes alignment restrictions for the byte offsets.

There is also the alignment restriction on CLONE. There doesn't seem to be
an alignment restriction on zfs_clone_range(), but maybe it is buried inside it?
I think adding yet another pathconf name to get the alignment requirement and
whether or not the file system supports it would work without any VOP change.

rick