Avoiding transcoding ...

[jellyfin]

That's expected behavior. MKVs are not widely supported by all clients to begin with. MP4 are, so there's way less of a chance that MP4 containers will cause a remux.

All people I ever met in person or online rips their DVDs and BluRays with makemkv, mostly because it is the best container.

I find it strange that such an important container type is not supported by a client for a server that services such rips - it seems to be the most logical thing to FIRST support MKV, simply as most content will be available in this container format.

I never bothered before, as I locally just use VLC to play anything.
But after I found Jellyfin, I liked the idea of such a server - and even a way to watch from remote.
But I cannot do that with such an amount of remux files ...

That means, either ditch Jellyfind or re-encode ALL my stuff as MP4, loosing most subtitle track.
I will need some days to get over that.

What Jellyfin is doing is remuxing your MKV container to MP4 so that the client can play it.
This is not a bug. The client just can't handle MKV containers most likely.

MP4 can only handle ONE subtitle track, which makes it ... totally unusable.

Thanks for the explanation!

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Just to clarify, remuxing is not the same as transcoding. Remuxing when a client reports it is not compatible with the container is the expected behavior and remuxing uses *very few* cycles (i.e., small amount of CPU or GPU). Transcode/remux files in cache are temporary and cleared out on whatever schedule is set in your Scheduled Tasks. I would not fret about this, very much the expected behavior.

Yes, I know - but constant creation of immense amounts of new files is nearly as bad, just the CPU usage and time is smaller without transcodes.

You may not know, but constantly writing new files in a large number and in terms of total size is surely going to kill your disk, regardless if HD or SSD - and your whole Mac with M* CPU, as normally you cannot replace it's disk.

It is totally unacceptable to me (and sure most people) that such files get created - I need to remove both transcoding and remuxing!