/var/lib/jellyfin/transcodes

[TheDreadPirate]

Thanks for your advice.
But I don't think any of this explains why the issue never happened before. My library hasn't been grown for months. If 2-3Gb per min is normal, then ppl need 180G for one 3 hrs movie. Then what happens next? Will Jellyfin at least automatically delete all files from this location? Yes, version is 10.8.13

You're talking about the transcode directory.
The rate this directory grows entirely depends on the source that's being transcoded. A transcoded video needs to be stored somewhere on the server so that it can be streamed to a client.

When a client is done with the transcoded stream, the transcode should be automatically deleted (in most cases).

You state that your transcode directory has access to 8GB. That's probably fine for 30 minute episodes. But for things like 4K movie transcodes that are hours long, that may be too small for the transcode to be stored.
Also, if you have multiple people transcoding at once, all that data is being stored in the same place.

Generally, storage has gotten cheaper over the years. It shouldn't cost you too much to upgrade the storage size for your Jellyfin server. 8GB is just to small for some cases when it comes to transcodes.

No, I don't agree what you are saying, 'cause I have been streaming for long time w/o any issues. Yes, 2-3 hrs 4k movies, no disk space issue.
But hey, free is free, it might come at a cost.

You may not have been transcoding before.  We don't know your history, what kind of devices you have, nor what codecs your media is encoded with now vs before.

If you've only recently added 4K movies that is, I'm assuming, also HDR, keep in mind that HDR compatibility is inconsistent between devices.

If the original file is 70GB (typical for 4K HDR video) the transcode will be, at least, that big.  Probably bigger if you are stepping down from HEVC to H264.

Regardless of WHY it's happening now and not before, it's happening now.  I gave you your options, of which #2 doesn't apply to 10.8.13.  Expand the volume /var/lib/jellyfin is located on or change the directory that transcodes are written to in Dashboard > Playback > Transcode path.