I think you should lower your `dirty_background_ratio`, 'cos currently kernel will start writing down dirty data from the cache when 20% of `Active` (as reported by /proc/vmstat) is filled,
on 128GB server this can be a lot. You should base your settings on your IO subsystem capabilities — somewhere around the size of the cache of your RAID array.
If you really have IO issues, consider checking more kernel parameters:
sysctl -a|egrep '^vm.(dirty|swap|over)'
and also check `pg_stat_bgwriter` data, it shows accumulated data about IO performed by checkpoints, bgwriter and individual backends.