Can not find what using my memory

Hi, i have a question. I have vps with Debian 10, ISPConfig and 8 GB RAM, total 7978 and 2 websites. When i start my vps or do reboot, after using command free -m i can see something like

Code:

           total        used        free      shared  buff/cache   available
Mem:           7978        1444        6450         45         267        6256
Swap:             0           0           0

For example about few hour i can see free for example 5200. Tried to use command to clear
sync; echo 3 > /proc/sys/vm/drop_caches

After that i get free from 5200 for example to 5900, or 4900 and after clear free 5500. But never get to 6400 again, only if i do reboot of my server.

Any idea what should i check? Using top command can see in first lines

Code:

1557 web7      20   0  136220  34856  27008 S   1.7   0.4   0:02.75 php-cgi5.6
 1483 www-data  20   0  353088  26476  13728 S   1.0   0.3   0:00.54 apache2
  665 root      20   0  841328  23908  10016 S   0.7   0.3   0:03.78 fail2ban-server
  798 mysql     20   0 1608196 174528  17772 S   0.7   2.1   0:13.01 mysqld
   10 root      20   0       0      0      0 I   0.3   0.0   0:00.37 rcu_sched
   35 root      20   0       0      0      0 I   0.3   0.0   0:01.32 kworker/1:1-events_power_efficient
 1459 www-data  20   0  353088  26596  13768 S   0.3   0.3   0:00.59 apache2

I am new with vps and debian. But it looks like i have to optimze mysql and maybe fail2ban? Because in ispconfig i can see a lot of attempt with ban or unban ip address.

Thank you.