ARC is completely separate from FS caches... if the kernel needs memory, it will tell ZFS to prune the ARC, however it's not exactly instantaneous. Newer vers

ARC is completely separate from FS caches... if the kernel needs memory, it will tell ZFS to prune the ARC, however it's not exactly instantaneous. Newer versions of htop also now have counters for ARC usage (compressed or uncompressed)... but it still shows up as used rather than cache.