Scale-out storage: Proprietary? commodity? Or both?

Depends what the object of your quest is...

There is no doubting that scale-out storage is now thoroughly achievable. Indeed, scale-out is pretty much the norm for object storage, plus it is now the primary option when it comes to huge volumes of file-based NAS, where scale-out means a highly-scalable file system that supports clustering. It is growing in popularity for block storage too."