The situation is this: I have SABNZB downloading movies (Blu-Ray,DVD,mkv). Initial download is to the cache drive, files are checked, evt. repaired and unpacked on the cache drive. Then during the night they are moved to their destination. Now when I want to watch a movie that I just downloaded while SABNZB is still downloading, repairing and unpacking movies that are still in the que, I get very choppy playback: few seconds playing, pausing.

Is there anything that can be done about this? Is it because the cache disk gets accessed by SAB while watching a movie. Is it lack of memory or processing power? It's a Core2Duo E6600 with 4 GB memory, Unraid 5.0b12

If its processing power then you should experience the problem on all drives.. Maybe the disk is indeed accessed to much ?  It sounds a bit farfetched (don't have the issue myself).

 

However.. The cache drive will not do much use with respect to speed in downloading.. Why don't you download to a different disk to see if th echoppy playback migrates to the new disk or indeed is gone ?  This will give you more info on where the issue lies.

The newly downloaded movie i try to watch is still on the cache drive. When at the same time SAB has finished downloading and is extracting a new movie on the same cache drive I notice choppy playback.

Sounds like the result of a lot of write action, maybe combined with used cpu and the read actions for your movie.. As said, move the movie to another drive and see if it still happens.

 

If yes: Its cpu

If not: Its the double disk action.

 

I suspect it will the second, easily solved by using another drive as your download location.

After the movie or DVD ISO has been moved to its destination in the array, it plays fine. The share where the movie is finally saved is set to use the cache drive. So it will initially be put on the cache drive. When SAB is unpacking it will do this on the cache drive which means lot of write-access to the cache-drive. This will influence the playback of a movie.

Sounds like you know your answer then 

Sometimes it helps just to think it over and reading a bit here and there. It's easy to overlook something.