Do your machines do compute heavy loads or something?
For a DB I'd expect a lot more storage per node
a 1U server has capacity for 8 drives, we used 2 slots for the OS (RAID1), 2 slots for the WAL volume (2 slots) leaving only 4 slots in RAID10.
So I'm already cheating a little and claiming WAL storage was part of total storage.
I doubt you're saturating the PCIe bus bandwidth on any of them?
I imagine your DB is extremely high performance, though!
Do your machines do compute heavy loads or something?
For a DB I'd expect a lot more storage per node