Excuse my presumption if you already know about this, but there are ways to make the I/O less aggressive for processes that hit the disk hard.
Since your graph header says 'linode', I'm assuming your servers all run Linux, and you should take a look at ionice[1]. It adjusts the IO priority of a process like 'nice' adjusts CPU priority. For example, prefixing your call like this: "ionice -c 3 tarsnap...." will give tarsnap the lowest priority access to the drive.