PBS sync job (pull) not using full available BW

[broth-itk]

Dear Support Team,

my offsite PBS server has a sync job configured in order to pull backups from another site.

The maximum bandwidth of the job is only around 100MBit/s.

A test with iperf3 returns the full available bandwidth of around 220MBit/s between both sites.

iperf3 -c 10.200.xx.xx -t 500 -R -l 1350

Note the packet length of 1350.

Repeating the test without the -l argument, the result is almost exactly what I see with PBS sync (100Mbit(/s)

iperf3 -c 10.200.xx.xx -t 500 -R

Obviously there look to be issues with packet length.

Setting the MTU on both sides to a low value (LXC containers each) does not work.
When setting the TCP MSS clamping to a lower value, I need to use smaller packet lengths on iperf as well to obtain the desired result (directly related).

Running iperf with multiple streams, I can get the full BW as well:

iperf3 -c 10.200.xx.xx -t 500 -R -P 2

Questions:

- How can I instruct PBS to use a smaller packet length (if possible)
- Since MTU and/or MSS Clamping does not help, are there other options?

The parallel option from https://lore.proxmox.com/pbs-devel/20250318122423.385684-1-c.ebner@proxmox.com/T/ could help but since iperf is able to use the full B/W, it would work as-is.

Any feedback or comments are very welcome.

Thanks!

Best regards,
Bernhard

 

[Chris]

Is this a followup to the discussion in https://forum.proxmox.com/threads/backup-sync-performance-over-vpn.164450/? The parallel sync option is currently not pursued further because of reasons mentioned in this thread on the mailing list, https://lore.proxmox.com/pbs-devel/850bd324-9251-4f69-9526-5cf6759bebd6@proxmox.com/

 

[broth-itk]

Not exactly a followup since the root cause of the performance issue was solved with BBR and sysctl tuning.

But your intentions are good- we've upgraded our internet connection and have 200MBit/s now

Contrary to my expectation, the sync B/W was still 100MBit/s and I did take a deep dive into the rabbit hole.

After hours of scratching my head, I discovered that the problem is reproducible with iperf3 and can be fixed by setting a smaller packet length.
Since this setting is unavailable in PBS, I tried to solve it by adjusting the MTU settings of the container or MSS Clamping on the firewall.

Both not yet yielding great success.