rsync - Aug 2023 - How to tune rsync to speed up?

Hi there,
we're facing some flapping traffic when rsyncing atm 70T from one server to
an DELL Isilon.
Both systems are connected with 10G Fiber (not Channel).
So we started with one simple "rsync -a /src /dest" to the DELL by
using
NFS3.
So it runs with around 3Gbit for some seconds, than it were 30Kbit for also
some seconds and again "some" Gig were transferred and than again it
decreases to 50kbit.
This was all the time for about 24 hours.
Than (this is actually not my doing, it's one of our researchers) i send
him a little pythonscript to use more rsyncprocesses. He modified it and
now it's around 5Gbps and 10Gbps.
So this helps a lot, but now I'm wondering what's the reason, because I
always thought (and had observed it so far with rsync) that it makes full
use of the network card (if the data is provided accordingly)
Some thoughts about that?

Here some specs:
DELL Server :
7515 with 1T RAM, sfp+ nic,
Software: opensuse leap 15.2 , rsync 3.1.3

DELL Isilon:
sfp+ connection, OS: 9.5.0.3

Thanks Sebastian

-- 
Mit freundlichen Gr??en
Sebastian G?decke
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Sebastian G??decke via rsync <rsync at lists.samba.org> wrote:
> We're facing some flapping traffic when rsyncing atm 70T from
> one server to an DELL Isilon.
> Both systems are connected with 10G Fiber (not Channel).
> So we started with one simple "rsync -a /src /dest" to the DELL
> by using NFS3.
> ...
> I always thought (and had observed it so far with rsync) that it
> makes full use of the network card ...
If you are using NFS, it is NFS -- not rsync -- which is managing the
network traffic.  In such a setup rsync will perform about as well as
cp(1).