ZFS Replication high network BW on sender

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BrentI

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Nov 29, 2014
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Hello

I have an odd problem where we replicate between 2 FreeNAS 9.3 appliances. They are located remotely with 50-100Mbps of bandwidth between them.

However for some reason even though I have set bandwidth limits to be 5000KB/s at the senders end it generates 200Mbps of traffic which gives the router at that end a real hammering.
And at the receiving end around 40Mbps of traffic flows in.

Replication completes fine, but I just want to understand why and fix the reason why it is generating 200Mbps of network throughput at the sending end.

Attached are screenshots from the reporting tab showing the network bandwidth.
Any help would be greatly appreciated.
freenas-receiver.JPG
freenas-sender.JPG
 

cyberjock

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Mar 25, 2012
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Well, I don't have a good answer for this. I just checked one of my systems that replicates. I don't have a bandwidth limit specified and my two ends are local to each other, but they track virtually identically. I'm not sure what the problem is exactly, but are you sure your network infrastructure is properly configured and you don't have some kind of weird network setup that is causing odd traffic patterns? I tend to think this is a misconfigured network, but I'm really just guessing because I don't have any idea how I could deliberately create this condition, let alone accidentally find it. Is your router's outgoing traffic consistent with what em0 is receiving? Could it be the router is sending craploads of retransmit requests? Could it be that your router is telling the box to stop and start transmitting so often that it's amplifying the output?

Can you provide a debug file? Can you provide the output of netstat -i?
 

BrentI

Dabbler
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Nov 29, 2014
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Just wanted to update you that this is now solved. It was due to a very large number of resends as cyberjock had suggested.
We were getting lots of packet-out-of-order messages requiring retransmits which made up the huge traffic.
Have moved traffic over an alternate link and it is happily running at the throttled 5000KBps.

Thanks for your help.
 
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