I have a few DL380 G7, so I'm going to presume that the built-in RAID controller has no option for JBOD, like mine. You've got two options for RAID on these HP blades when using Proxmox/Debian -
1) Set each drive to be a single RAID 0 on the controller so that the OS will see each one, and then software RAID these individual drives.
2) Buy a separate RAID controller that does support JBOD and ignore the integrated controller altogether.
I went with option #2, as the first one seemed more of a hack and (presumably) harder to maintain. I got a few really nice Adaptec PCIx4 cards for $30 apiece through Ebay. Let me know if you want the specifics.