Constant hdd write but iotop shows nothing

The disk activity monitor widget in KDE (Debian) shows constant HDD write around 12 MiB/s, when I run iotop, there is nothing that would be constantly using HDD. When I run atop, at first PAG is red and blinking but after about 3 seconds disappears, when i run free -h, I get: total used free shared buff/cache available Mem: 7.7Gi 2.2Gi 3.0Gi 1.1Gi 2.5Gi 4.2Gi Swap: 7.9Gi 0.0Ki 7.9Gi Any idea what can be causing this or how to find out? Also, i tried to clear the cache, it cleared to 1.5 Gi but after less than 5 minutes it was back to 2.5 Gi as shown above. Also i am thinking that Debian is using quite a lot of memory given that only firefox with the stackexchange window is open.