how control the amount of back buffer (cache)?

[geextahslex]

Hi, I would like to know how to control the red part of the cached data in this picture?
The total amount (green color) is demuxer-max-bytes=800MiB
The yellow forward part (yellow) is demuxer-readahead-secs=60
but which value controls the (red) back buffer part?

Thanks for any help :)

edit:
okay I think it is about

demuxer-max-back-bytes=300MiB
demuxer-donate-buffer=no