There are several threads with the same problem:
https://forums.unraid.net/topic/27009-buffering-issues-while-running-parity-check/
https://forums.unraid.net/topic/90488-parity-check-cause-plex-network-issue/
https://forums.unraid.net/topic/95676-plex-stuttering-while-moving-files-from-one-hdd-to-another/
I really wonder why the impact is so huge, although the write speed is under 10 MB/s as you can see in this video (movie is located on Disk 1, which is in the 2nd row):
unraid plex judder 720p.mp4
This happens for parity checks, parity builds, mover actions and usual file write operations.
I did not tested reconstruct write, but I think it could be related to the read/modify/write process which interferes with the reading of the movie through Plex. The strange thing is, that even transcoding does not help. I thought it would, because Plex transcodes the movie in advance and fills a buffer (I'm using ram transcoding), but the behaviour is completely the same with and without transcoding.
Is it possible to limit the write speed of the parity disk (to like 50%)? I think this could solve the issue as the other disks would throttle, too. My idea was to start a dd read process to /dev/null from the parity disk every time the parity disk has open write transfers.
Or would it be possible with /etc/security/limits.conf?