The New Class: NVMe and SOP
Don’t Get Stuck Riding A Dinosaur: Next-Gen Interfaces And Protocols ExplainedIn March 2011, NVM Express Work Group announced version 1.0 of the NVMe standard. NVMe offers a streamlined queing interface, command set, and features that sound a lot like SCSI Lite. However, where SCSI doesn’t extend to SATA drives, creating a market split between SATA and SAS, NVMe is designed to extend to both ends of the enterprise storage spectrum, from client drives to external arrays and server caching. Because NVMe has a smaller command set than SCSI, it could be a boon to solid-state drives by having a lower overhead than SCSI. Time will tell.
Practically growing up alongside NVMe, SCSI Over PCI Express (SOP) is the work of the ANSI T10, which is the committee responsible for the SAS interface. NVMe and SOP also share another (somewhat obvious, if you look at the name of each) trait: both route commends over the PCI Express bus, which affords storage drives ample bandwidth and makes the SATA interface especially look like Grandpa’s storage interface.
Now, routing SCSI over PCIe isn’t strictly the wave of the future. After all, LSI, Marvell, and PMC (to name a few) have all introduced to solutions that do just this. Unfortunately, in these cases, there was a catch: You had to use proprietary drivers for all of these vendors’ implementations. If you wanted SCSI over PCIe on agnostic, universal hardware, your choices were zip, zilch, and nada. Thanks to SOP, though, you can have your cake and eat it, too.
SOP has a partner in crime, too, and together they’re a dynamic duo. The other half of the equation is a new storage interface technology called PCI Express Queuing Interface. SOP and PQI work hand in hand. SOP defines a transport protocol for SCSI commands, and PQI determines how the commands are queued and managed, registers are loaded, and storage traffic is managed across the PCIe buss. As a pair, SOP and PQI are commonly known as SCSI Express.

