SCSI Standards and Technology Update
September 21, 2015, SNIA Storage Developer Conference, Santa Clara, CA—Rick Kuticipal and Greg McSorley from the SCSI Trade Association provided an update on the SCSI standards. The standards are evolving along with the many changes in the storage technologies.
The latest additions to the SAS standards and roadmaps include 12 Gb/s SAS, advanced connectivity, Multi-link SAS and u.2, and the developments in 24 Gb/s SAS. Other new features include storage intelligence and zoned block commands (ZBC) for shingled magnetic recording.
The SAS protocols are used with many devices, from SAS fabrics to direct attached storage to external storage and down to the underlying drives. Almost everything associated with storage including switches, expanders, etc. use the SAS protocols. The interface has gone through a number of iterations and has increased data rates from 3 to 24 Gb/s since ’04. The latest revision handles up to 12 Gb/s with the SPL -2 and -3 extensions for transmitter training, enhanced power control, persistent connections and other SSD enhancements.
Now, the standard is addressing drive management or shingled magnetic recording to hide al SMR issues. New extensions for host managed functions address new device types and add new zoned commands that require sequential writes. The host aware functions are a superset of the drive managed and host managed commands where sequential writes are preferred. The ZAC and ZBC commands are expected to be completed by the end of the year.
The move to 12 Gb/s called for different Phys and SAS-2.1 was published in ’11 to manage connectivity and identify high-density connectors with both active copper and optical support. SAS-3 released last October described the 12 Gb/s SAS interface, transmitter training, a x4 backplane connector and an optical mini-SAS HD connector.
The newly defined connectors simplify cable and connector options with double the density. The new connectors are the same for passive, active, and optical cables, with the ability to supply power for the active copper and optical cabling. The active copper solution is capable of 20 m reach while the optical cables can go to 100 m. The cables are managed through our of band signals and the built-in intelligence communicates with the rest of the systems via I2C signals. Now, a single host bus adapter (HBA) can support passive copper, active copper, or optical cables. See figure 1.
The transfer capabilities and protocols will be the limiting factor in determining performance in the next generation of storage devices.
SFF-9401 provides recommended pin-outs for bridging older SAS-2 to SAS -3 and -4 connectors. The new standards also define SAS device to id-plane interconnects with various combinations of 1x, 2x, and 4x ports to support standard dual-port, multi-link, and multi-function ports.
The u.2 form factors include definitions for up to 25 W of power, cooling, vertical and horizontal connectors, functions to manage accessibility and serviceability in traditional drive form factors. The power can be used for both SAS and PCIe devices and use the same connector and PCIe electrical specifications. The intent is to make PCIe storage the same as legacy enterprise drives and also to meet the SSD performance demands. See figure 2.
The protocols and services can be accessed across the various connector types.
The gen 4 standard will address 24 Gb/s bandwidth while providing back compatibility to gen 2 and 3. The intent, to the extent possible, is to retain existing tools and test equipment and the underlying SAS reliability, scalability, flexibility, serviceability, and manageability. The specifications should be published by mid ’17 for platforms to launch in ’19.
The thumbnail characteristics for the 24 Gb/s SAS include:
22.5 Gbaud, 128b/130b encoding, 20-bit forward error correction, and protocol and block-level enhancements for SMP, storage intelligence, and zone block commands. In-flight changes will address SAS-4 transmitter training algorithms for continuous adaptation, and new fairness enhancements. The SNIA posts updates on the various standards activities at www.t10.org and organizational information at www.scsita.org
