Does the OMV has any sort of hybrid raid similar to the SHR-1 or SHR-2(Synology hybrid raid) ?
SHR-1 IS RAID1+5
SHR-2 IS RAID6
The main advantages of this type of config is that you can use diferent disk sizes and maximize the usable disk space.
If you have 3 disk with the same size in SHR-1 or raid 5 the usable space is the same .
But if you have 3 disks, 1TB,2Tb,3Tb with SHR-1(raid1+5) you will have 3Tb of available disk space, while with raid 5 will only have 1tb.
I believe this feature would help a lot of users like me who have disks from 250Gb to 2 Tb(2x 250Gb,2x 500Gb,3x 1tb,3x 2Tb) this way we could use diferent size disks and not lose so much space like in a normal raid 5 or 6. 
Here you can understand better what i am talking about here.
http://www.synology.com/support/RAID_calculator.php