How to increase RAM buffer for disk writes

I have 8GB of RAM but OMV seems to use only 2GB as a disk write buffer. My disk filled up to 50% is pretty bad at writes with 50MB/s or even 30 with small files. So first ~two gigabytes goes really smooth at full 1Gb connection and then it drops hard. I could really use full RAM potential for this.

Ok i'll provide more info. My disks both are 2,5" and 5400rpm so not so great in performance (SMR technology makes it even worse). If the disk is empty, write speed at the edge of the disk platter exceeds 100MB/s so sending a file (8GB) over local network to my NAS looks like this:

And the output of free -m during write operation - so pretty much the whole available memory is filled up quickly.

              total        used        free      shared  buff/cache   available
Mem:           7385         193         117          29        7074        6861
Swap:          7608           0        7608

But the same operation on disk filled over 50% with data looks like this:

Win10 graph clearly shows that only first ~2GB lands in cache, the rest is slowly written to the disk. A the first falling slope free -m shows around 2,5GB in the "buffer" column, then it fills up slowly reaching over 6GB at the end.

Here i don't know why its not taking data straight into buffer, but it waits for the disk, filling the buffer along. It uses the buffer more as a fast-read-later-cache instead of a write-fast-now-cache.