Use buffer instead of cache if possible

I am doing join and bitwise and on a huge data. Monetdb caches 61 GB of ram and writes 152GB on disk. I use dstat for resource monitoring. I see that buffer is not used at all, monetdb just caches memory and used reads/writes. I know monetdb uses mapped files, but is there a possibility to use buffer more than caching? I was not able to find any information how to force process use more buffer. P.S sorry for stupid question, I am not very strong in linux stuff. Thanks in advance.