Re: [nfsv4] Is NFSv4.2's clone_blksize per-file or per-file-system?

On Sun, Aug 10, 2025 at 7:32=E2=80=AFAM Rick Macklem <[email protected]=
m> wrote:
>
> On Sun, Aug 10, 2025 at 6:58=E2=80=AFAM David Noveck <[email protected]=
m> wrote:
> >
> >
> >
> > On Sat, Aug 9, 2025 at 5:02=E2=80=AFPM Rick Macklem <rick.macklem@gmail=
.com> wrote:
> >>
> >> On Sat, Aug 9, 2025 at 1:12=E2=80=AFPM David Noveck <davenoveck@gmail.=
com> wrote:
> >> >
> >> >
> >> >
> >> > On Friday, August 8, 2025, Rick Macklem <[email protected]> wro=
te:
> >> >>
> >> >> On Fri, Aug 8, 2025 at 8:38=E2=80=AFPM Trond Myklebust <trondmy@gma=
il.com> wrote:
> >> >> >
> >> >> >
> >> >> >
> >> >> > On Fri, Aug 8, 2025 at 9:47=E2=80=AFPM Rick Macklem <rick.macklem=
@gmail.com> wrote:
> >> >> >>
> >> >> >> Hi,
> >> >> >>
> >> >> >> I'm looking at RFC7862 and I cannot find where it
> >> >> >> states if the clone_blksize attribute is per-file or
> >> >> >> per-file-system.
> >> >> >>
> >> >> >> If it is not in the RFC, which do others think it is?
> >> >
> >> >
> >> >  Before you told us about ZFS,  I would have assumed per-fs.
> >> >
> >> > Given the uncertainty in the spec, you may wind up dealing clients t=
hat assume it is per-fs.
> >> >
> >> > Although this is not a  catastrophe, you might want to file an errat=
a report explaining the negative consequences of assuming this is per-fs. I=
t won't get into a spec for a long while but it does provide as much warnin=
g as you can right now .
> >> >
> >> >
> >> >
> >> >>
> >> >> >> (Or maybe, if you have implemented CLONE,
> >> >> >> which does your implementation assume?)
> >> >> >>
> >> >> >> In case you are wondering why I am asking,
> >> >> >> it turns out that files in a ZFS volume can have
> >> >> >> different block sizes. (It can be changed after the
> >> >> >> file system is created.)
> >> >
> >> >
> >> > The guy who allowed that probably thinks it's a helpful feature.  Si=
gh!
> >> It's not just a feature change after creation, it turns out to be base=
d
> >> on file size as well.  A small file gets 512 and a larger one gets a f=
ull record
> >> (128K on my test system).
> >>
> >> And, yes, block cloning requires alignment with 512bytes or 128Kbytes
> >> depending on the file.
> >>
> >> I can return 128K for clone_blksize and that will (sub-optimally) hand=
le
> >> the 512byte case, but I think it is also possible to increase the reco=
rd
> >> size from 128K-> after the file system has files in it.
> >>
> >> I'll take a look at the Linux client to try and see if/how it uses
> >> clone_blksize.  I need to decide if I should always return 128K
> >> (or whatever the full recordsize is) or 512 for the small files.
> >
> >
> > I don't see the point of returning anything but 128K given what you sai=
d above.
> > If a file has to be smaller than 512 to merit the 512 block size, it co=
uld still be cloned with a 128k clone_block_size.  The spec makes an except=
ion for the last block of a file being shorter than the block size so retur=
ning a 512-byte clone_block_size.
> I'll be experimenting with it soon.
> What I do not know (you could write what I know about ZFS on a
> postage stamp;-) is whether the blksize for a file changes as it
> grows.
> --> So the problem is a file might get 512 because it is small when
>      first created and then grow large. Again, I do not currently know
>      what determines the blksize. Whether it is the first write being les=
s
>      than a record size when created or maybe it does switch to recordsiz=
e
>      (128K in my case) when it grows beyond 128K or ???
>      - I do know that ZFS allocates new blocks whenever data is written
>        to a file, even if the file is not growing. (Which is why it canno=
t
>        support ALLOCATE at this time and probably never will.)
>
> I'll be poking at it. For now, I just do not know, rick
I should have done a scan before posting.
I just ran a little program that printed out the blksize of every
regular file in a ZFS file system.
It turns out that the blksize is any exact multiple of 512 up to
128K (the record size for the volume).
Since most are C sources or objects, most are less than 128K.

If I return 128K, then most files would not be CLONEable unless
the CLONE is for the entire file.
Of course, I do not currently know how clients actually use
clone_blksize either. (Do they check alignment using it before
doing a CLONE or ???)

I'll be playing around with CLONE for both FreeBSD and Linux
in the coming days.
I'll post if/when I have useful info, rick

>
>
> >>
> >>
> >> Thanks for the comments, rick
> >>
> >> >
> >> >> >>
> >> >
> >> >
> >> >>
> >> >> >> Thanks, rick
> >> >> >>
> >> >> >
> >> >> > Yes, but since ZFS only supports filesystem level snapshots, and =
not actual file cloning, does that matter to anything?
> >> >> ZFS now has a feature it calls block cloning, which does clone file=
 ranges.
> >> >> (It was only added recently. I do not know if the Linux port uses i=
t yet?)
> >> >>
> >> >> rick
> >> >>
> >> >> >
> >> >> > Cheers
> >> >> >   Trond
> >> >>
> >> >> _______________________________________________
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