SMB traffic causes periodic network reconnects

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Oct 13, 2015
\Edit: I spent a whole day on searching the issue and just when I finished this post, I found the solution. The problem is with my Windows PC. Everything works fine when I use another PC. I was so focused on the issue being the NAS since it was the NAS reporting dropped ethernet connection but aparrently these where caused by faulty pakets from the PC. Hopefully at least the experience will help someone else.

I had my NAS running for quiete a while and am currently on Freenas 11.1. It uses 16GB of DDR3 RAM alonside an AMD A10-5700 CPU and an 8port LSI controller for two Zpools.
Yesterday I tried to copy some TB of Data over SMB-Shares to my Windows 10 machine. During that, i had to restart my home router. And after that the copy process caused the network connection of my Freenas to drop about every 10 seconds and rebuilds right after. When I pause the copy process, the dropping stops. Also when I have low transfer rates below 30 mbyte/s it also doesn't drop. But as soon as the transfer rates reach the 50 - 100 mbyte/s, the connection will drop repeatedly.

This happens for both Zpools. Writing huge files to them via SMB doesn't cause drops but has a way more instable transfer rate (before 80-90 mbyte/s, now 10-60mbyte/s). So I thought, I might have corruptet some system files with the sudden router restart and made a fresh install on a new stick. But again the drops happened. Digging deeper, I noticed that FTP-Downloads and Plex-Streams don't cause drops, so this seems solely related to SMB. Another weird thing I noticed alongside the way: FTP-Downloads over Firefox reach max speed with 90mbyte/s but Filezilla peaks at 25mbyte/s and WinSCP & Cyberduck are even worse.

Alonside several restarts I also did a router reset to get rid of faulty DNS entrys, exchanged the cables and also tried a direct ad-hoc-connection between PC and NAS without the router. None of these solved the issue.

So it almost seems I somehow fried my server's NIC. But how comes that only SMB traffic is affected? Are there any log-files that may give a solution? And is it possible to alter the SMB packet size since the problem only occurs with high data rates? And does someone have a clue why Filezilla is so much slower than browser download?
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