I made some bad choices when defining the sizes of my Logical Volumes while installing OMV. After backing up about 3.5 TB of data I found I had to increase one LV and decrease another.
I found this how-to that explains how to do it: http://www.tecmint.com/extend-and-reduce-lvms-in-linux/
I did not quite follow the steps but used this how-to as a guide to resize. Worked like a charm
I must admit I used the OMV web interface to remount my resized shares
If your a noob (like me) this guide will surely help you.
HTH
Greetz
Sjonnie