https://oss.oetiker.ch/rrdtool/doc/rrdtool.en.html
apt install rrdtool
rrdtool dump name.rrd
rrdtool fetch filename.rrd AVERAGE
RRDTOOL Values NaN
rrdinfo filename.rrd ds[inodecache].min = NaN --> datastore-name:U = inodecache:U
rrdtool tune filename.rrd --maximum datastore-name:U --minimum datastore-name:Urrdtool tune zfs*.rrd --maximum lambda:U --minimum lambda:U