Why wouldn't you want
Buffering? It is a mechanism used to ensure smooth playback of media. The term has been misused when referring to buffering when using the internet which results in pauses and stuttering of video, but what people should really be saying is that the internet couldn't keep the buffer full, so they saw pauses in the video. Buffering is good. Using the whole buffer, or emptying it, is bad.
BTW Bob mentioned the Audio Buffer above but I'm sure you tried turning off the Video Buffer. That should stop the saturation of the network connection and the heavy disk activity, but I'm not sure it will stop the stuttering of the video. Was it the Video Buffer that you disabled?
Netlimiter is a way to stop the network saturation you are seeing, which is why I suggested it. But I wouldn't buy the software unless you can confirm that is the cause of the problem you are seeing.
A 40Mbit/s video file would be a Blu-ray movie, which an i3 should be able to play. But if the MC Server is trying to send it all at once, that could be 50GB of data. MC uses the Temporary Files location ( Options > File Location > Program Files > Temporary Files ) to store buffered video. Does that point to your m.2 SSD, and does it have 50 to 75 GB of free space? If not, maybe the SSD is being filled and interfering with the whole playback process. That might explain;
The other way round - if I click play on my client and the client receives the file from the server everything is fine.
Where the MC Server tries to push the whole file to the Client, which then tries to buffer the video, if the MC Client pulls the video from the Server I would guess either the Client is aware of how much space it has available for buffering, or it just pulls the video as required rather than try to pull the whole thing.
Does that sound right Bob?
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Actually, how are you pushing a video from the MC Server to the Client?
I don't usually have Media Network turned on in my MC Client. That still allows me to pull media from the Server and play it on the Client. But I can't push video from the Server to the Client, even when the Client is connected to the Server. The Client isn't visible as a target on the Server.
If I turn on Media Network on Client I am able to push a video from the Server to the Client using the MC Server interface. That isn't using DLNA. I tested this using a full Blu-ray folder structure rip on the server, which means that I have to have the Client set to convert Video to H264-TS 1080p as MC won't play my Blu-ray folder structure rips to a Client without conversion. I would need to convert the Blu-ray to MKV or something to show the original video and audio on the Client. I guess you are playing MKV or similar files to be expecting a 40Mbps network load?
With that setup I see a network load of about 8Mbps for a Blu-ray (Big Hero 6, Bitrate 40.5Mbps, total Blu-ray disc size ~40GB), and I see buffering in the Temporary File location on my Client, with 128MB files accumulating as the video plays, to a total of 3.86GB, which is consistent with an H264 movie.
In fact, as an experiment I used my MC Client user interface to push a video, which has been served from the Server the Client is connected to, back to the Server and have it play there. That works, but I see an audio synchronisation issue then, since the video is going Server->Client->Server. Just an interesting aside.
Anyway, that brings me back to your original question: Is my MC media network not correctly configured?
That depends on how you are trying to use MC.
Are you playing full Blu-ray rips in MKV format to the Client?
Do you need to push video from a Server to a Client? (Which is giving problems.) That seems to require Media Network to be turned on in the Client.
Do you have a Client connected to a Server, so that you could always pull video to the Client? (Which you know works fine.) That doesn't require Media Network to be turned on in the Client.
Perhaps summarise how your Media Centre installations and TVs are set up and connected, and how you are expecting to use them.