resizing rrd about munin

[niclan]

This is documented: https://github.com/munin-monitoring/munin/blob/master/doc/advanced/custom-rrd-sizing.rst

Can't really resize existing rrd files though: The simplest way to get the longer retention is to change the config and then delete the files you want to have long retention, because they will not be remade or resized automatically.

If you want to retain the data you need to use rrd commands to copy the data from the old small file to the new large file. I've never been motivated enough to go through with this myself.