openssh unix dev - Mar 2025 - Support for transferring sparse files via scp/sftp correctly?

On 3/4/25 05:34, Philipp Marek via openssh-unix-dev
wrote:
>> Does OpenSSH scp/sftp mode transfer sparse files correctly, i.e. are
>> holes skipped and not transferred as chunks of 0 bytes? [1]
>>
>> We're asking about sparse files in the >= 1PB range, which
consists of
>> multi-TB holes with around 600-2000GB of valid data.
> 
> 
> Perhaps rsync would be a good fit here,
> it supports --sparse.
> _______________________________________________
> openssh-unix-dev mailing list
> openssh-unix-dev at mindrot.org
> https://lists.mindrot.org/mailman/listinfo/openssh-unix-dev
I think one of the issues you are going to face is that SEEK_DATA and 
SEEK_HOLE don't seem to be currently supported under OpenBSD. Since 
that's the home OS for OpenSSH this could create portability issues. 
While you can get around that with the judicious use of defines it means 
that the feature set will start to shift between different OSes.

Personally I think it's a good idea and I may explore it for HPN-SSH but 
I think it's going to be a hard sell for the OpenBSD community.

Chris

p.s. sorry for not replying correctly. My mail seems to be having issues 
and this is the only email in the thread I've seen.

On Tue, 4 Mar 2025 at 21:22, Chris Rapier <rapier at psc.edu>
wrote:
>
>
>
> On 3/4/25 05:34, Philipp Marek via openssh-unix-dev wrote:
> >> Does OpenSSH scp/sftp mode transfer sparse files correctly, i.e.
are
> >> holes skipped and not transferred as chunks of 0 bytes? [1]
> >>
> >> We're asking about sparse files in the >= 1PB range, which
consists of
> >> multi-TB holes with around 600-2000GB of valid data.
> >
> >
> > Perhaps rsync would be a good fit here,
> > it supports --sparse.
> > _______________________________________________
> > openssh-unix-dev mailing list
> > openssh-unix-dev at mindrot.org
> > https://lists.mindrot.org/mailman/listinfo/openssh-unix-dev
>
> I think one of the issues you are going to face is that SEEK_DATA and
> SEEK_HOLE don't seem to be currently supported under OpenBSD. Since
> that's the home OS for OpenSSH this could create portability issues.
> While you can get around that with the judicious use of defines it means
> that the feature set will start to shift between different OSes.
OpenBSD unfortunately does not implement so many other APIs. But other
OS do implement SEEK_DATA+SEEK_HOLE, including FreeBSD, Linux,
Solaris, Illumos and even Cygwin. Even NFS has a SEEK to lookup holes
and data sections in files.
SEEK_HOLE+SEEK_DATA are also now part of the POSIX standard, so IMO it
is time to face the bug that sparse files are not handled correctly
and fix it
>
> Personally I think it's a good idea and I may explore it for HPN-SSH
but
> I think it's going to be a hard sell for the OpenBSD community.
Selling points:
- Users need it
- Sparse files were always part of UNIX
- It's part of the POSIX standard:
https://pubs.opengroup.org/onlinepubs/9799919799/functions/lseek.html

Ced
-- 
Cedric Blancher <cedric.blancher at gmail.com>
[https://plus.google.com/u/0/+CedricBlancher/]
Institute Pasteur