rsync - Aug 2023 - Consider POSIX_FADV_NOREUSE?

Hey rsync,

Seems like there was an effort a while back to make use of POSIX_FADV_DONTNEED
on linux [1]. Though the linux patches landed, apparently the rsync ones never
did.

Over a decade later we live in a different world. Linux is using the new-ish
MGLRU on many distros, and (~6.3+) even has support for POSIX_FADV_NOREUSE [2],
which was dismissed in prior discussions for lack of support.

Maybe it's time to re-evaluate the utility of posix_fadvise in rsync?

[1] https://lkml.org/lkml/2010/11/21/59
[2] https://lore.kernel.org/all/20221230215252.2628425-2-yuzhao at
google.com/T/#u

Cheers,
Ronan

On Fri, Aug 4, 2023 at 3:24?AM Ronan Pigott via rsync
<rsync at lists.samba.org> wrote:
> Maybe it's time to re-evaluate the utility of posix_fadvise in rsync?

I would be glad to see this option added to `rsync`, especially when
mirroring very large sources / targets and I don't want to trash the
file-system cache.

However, I think any of these variants (either "dontneed" or
"noreuse") should be opt-in, because I often use `rsync -n -c` to see
what would change, and then issue a proper `rsync -c` which would
benefit from the file-system cache.  (Also, "dontneed" might be much
more aggressive than "noreuse".)

(BTW, if one starts implementing `fadvise` and friends, then there are
options to tell the file-system that the whole file would be read,
etc.)

Ciprian.