Virtio network adapter bridge performance when uploading and downloading simultaneously

[tiredjohn]

I have a Windows gaming VM, with a fast NVMe drive in a single disk array providing the storage. I also have a gigabit internet connection, when I run a speed test it usually reports 95% of that speed. For the most part everything is working great. However, I noticed one quirk - when downloading a file over the internet that saves to the array, suddenly the network speed plummets - the download speed from the internet will max out at around 250 Mbit, and the network interface will be saturated. (Running an internet speed test during a big download like this will give me 10 Mbit speeds.)

 

So I'm scratching my head trying to figure out what could be the issue here. The NVMe speeds as tested in the VM report at around 2000 MB reads and 1000 MB write to the array, so no issues there. The Virtio Ethernet adapter is showing as connecting at 100 Gbps, so that should be plenty fast enough. I know that when a file is downloading from the internet and saving to the disk, it comes in over the Virtio bridge and back out to the SMB share on unRAID - but given that the incoming internet bandwidth is 1 Gbit and the outgoing SSD bandwidth must be at least 20 Gbit (based on the array speed tests), I don't even know where to start looking to solve this problem. Any pointers would be much appreciated. 

 

Edited January 20, 2020 by tiredjohn
Added detail