SAS (Serial Attached SCSI) storage systems I/O interface and its data transmission rate and interconnect types are expanding again. SCSITA, the SCSI Trade Association, has been marketing, driving and supporting the next generation SAS 4.0 running at 24Gbps per lane. The Industry standards group INCITS-T10 is working with www.SFFcommittee.org on developing a new higher-speed SAS 4.0 specification that has many connector and cabling options. SAS 24Gbps products will be in ramp up volume through 2017.
SAS 3.0 12Gbps per lane—or “i” they prefer to use, current generation products—are in high volume usage. The external HDminiSAS 4i and 8i cable assemblies and PCB receptacles are the primary interconnect type used. See the SFF-8643 and SFF-8644 for details. This external connector/cable type has been used a lot for other I/O interfaces like SGI’s NumaLink I/O converged interconnect system. The internal HDminiSAS 4x and 8x connector and cabling are listed on both SAS specifications. The external QSFP+ connectors/cables are also on the SAS 3.0 spec and have been used in several storage systems through the last few years. So will these users stick with using this family and its newer, smaller microQSFP connector/cabling system and will it eventually find its way on the developing SAS-4.0 specification?
SAS-4.0 24Gbps per lane specification has three cabling interconnect type options so far. Besides QSFP+ and a possible QSFP28 and microQSFP, it has the new internal MiniLink connector that is smaller than HDminiSAS. See SFF-8611 and SFF-8612 for MiniLink details. MiniLink seems to be the choice for many storage systems OEMs and the huge end-user, installed base of storage systems. MiniLink is also used for PCIe I/O applications and is called OCuLink for those. However, some OEMs and developers of storage systems appear to prefer the SlimLine internal connector and cabling which is also on the SAS-4.0 specification. See SFF-8654 for the 4i and 8i version connector details.
Internal micro-ribbon twin-axial cable assemblies are usually preferred as they can be routed inside the box easier than round cables and have less effect on airflow. These cables are shielded to minimize crosstalk and EMI issues. Internal cabling options can be used to support a cabled backplane design solution to keep the cost of a PCB backplane much lower. It will be interesting to see if external cabling versions of SlimLine and MiniLink cable assemblies will be developed and specified in a new revision of the SAS specification.
External SAS cable assemblies are available in passive, active copper and active optical options depending on reach lengths within the racks.
As hard disk drives are replaced with solid state drives, the storage I/O interface of choice may differ than the traditional SAS interface. The NVM SSD products are designed and implemented using other connector types. Just like the SATA storage I/O standard stopped using SATA connectors and went with SAS connectors supporting the SATA protocol, the SAS protocol may find itself going over PCIe connector and cabling types. So many native I/O interfaces and their wide variety of connector/cabling types are going through convergence process as most datacenter end-users want to use just one type for simplicity and lowest cost of implementation. It will be interesting to see if some users prefer using the very new RCx connector and cabling within the storage rack connecting drive bays to a SAS switch box. So many connector and cable assembly type options are helping to create a huge adaptor assembly business. See SFF-8024 and www.molex.com for connector options charts.
In an upcoming blog I’ll talk about developing SAS drive and backplane connectors.
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