CTO, Nimble Storage
Claims of performance benefits of connecting flash directly to PCIe rather than to a storage HBA are exaggerated, especially in the context of flash in networked storage.
The extra overhead of introducing a storage HBA is in tens of microseconds, which is dwarfed by other latencies such as flash access and network transfer, which are often in hundreds of microseconds. Shaving off a few tens of microseconds is not a noticeable improvement in the end-to-end latency for most applications.
On the other hand, using flash in a disk drive form factor brings significant advantages such as the ability to share it across controllers and to service it non-disruptively.