when does Nginx have unexpected write on disk?

I use Nginx to cache static files. the main problem is sudden changes on write on the disk.

nginx.conf:

http {
    proxy_cache_path /cache-data levels=2:2:2 keys_zone=images_cache:60000m inactive=2d use_temp_path=off;


    server {
        access_log off;
        listen 8080 reuseport;

        location ~ ^/storage(\d+)?\.my-domain\.com/ {
            # Configure the proxy cache
            proxy_cache images_cache;
            proxy_cache_background_update on;
            proxy_cache_lock on;
            proxy_cache_use_stale error timeout updating http_500 http_502 http_503 http_504;
            proxy_cache_valid 200 2d;
    }
}

the main problem: this is a report of node exporter of Kubernetes (I have only an Nginx on this node)

I know:

• the read rate on the disk is not changed.
• the request rate is not changed.
• the hit/miss/stale rate is not changed.
• the volume size is not changed.
• the access_log is disabled.

some update:

• ram: 60GB
• capacity of the pod (cache-data): more than 200GB