Hi, I familiarizing with Rockstor and stumbled on the problem:
Trying to copy a large amount of data on an NFS share, and memory somewhat leaked from free to cached.
Memory is not released until restart. I thought that the 48Gb of memory enough for me, but it turned out that it is not. 
Having a large cache block is not a bad thing.
RedHat :
“Linux always tries to use RAM to speed up disk operations by using available memory for buffers (file system metadata) and cache (pages with actual contents of files or block devices). This helps the system to run faster because disk information is already in memory which saves I/O operations. If space is needed by programs or applications like Oracle, then Linux will free up the buffers and cache to yield memory for the applications.”
So, in fact, this is normal behavior. Thanks for answer. 
