The Anatomy of Serial Attached SCSI

Author: Harry Mason
LSI Logic

Serial Attached SCSI (SAS), much like legacy SCSI, is architected with the following components:

  • Host Controller:
    • ChipBoard
    • RAID
  • Expanders
    • Edge Expanders
    • Fan-out Expanders
  • Interconnection schemes
    • Connectors
    • Cables
    • Backplanes
  • Storage Devices
    • Disk Drives
    • Tape Drives
    • SATA Drives
  • Text/Validation Systems

The articles in this issue (Issue 5) and the following issue of Serial Storage Wire newsletter will break out each of these areas of SAS anatomy and discuss their various components and operation.

In this issue you’ll find an article from Maxtor describing SAS storage devices, and article from Adaptec explaining SAS expanders and an interview with PMC-Sierra that demystifies how the various SAS components may be interconnected with cables and connectors. Hitachi Global Storage will round out this issue with an insightful IT interview.

Issue 6 continues with the Anatomy of SAS theme, and will feature an article from Seagate on how to choose the best storage device for your SAS implementation. An Intel article will dive into more detail on which elements to consider when choosing your SAS host controller hardware and two development pieces from I-Tech and CATC will describe the considerations given to testing various implementations to insure robust and interoperable SAS solutions. HP will round out Issue 6 with a development corner article focused on implementing SAS backplanes.

In describing how SAS systems are constructed, we begin with the SAS Host Controller. The SAS Host Controller, generally implemented as a multi-ported controller, may reside on a server motherboard or alternatively be added into a system as an optional host bus adapter (HBA). HBAs are designed for the specific system expansion bus, such as PCI or PCI Express.

Host ports may be routed to either internal or external SAS connections. OEMs typically allocate a fixed number of SAS ports both internally and externally.

While some server manufacturers may elect to implement simple SAS connections to drives or external storage solutions, others may implement SAS RAID controllers. These RAID implementations, like simple host controller solutions, may also exist in the form of a single IC component or a more complex HBA system expansion card.

Port counts may be added with the use of additional controllers or through various types of SAS expanders. Expanders, much like traditional SCSI expanders, are used to “expand” the system architecture. Edge expanders are used at the “edge” architecture and are usually limited in their expansion capabilities. Fan-out expanders are needed (in conjunction with edge expanders) to take full advantage of the addressing capability of the SAS architecture.

SAS expanders may be designed as fan-out devices or as limited area fabrics that allow SAS to scale to dramatically larger system storage solutions. A single fan-out expander enables connection of up to 128 edge expanders and thus 16,256 SAS devices per SAS domain. Expanding storage in or near the server is essential to accommodate the growing demand for storage.

Host ports or expander ports, like legacy SCSI connections, can be routed through backplanes or through cabling schemes. SAS cable options allow for the use of low cost cables similar to SATA cabling for single port connections or Infiniband-style cabling may be used when wide SAS ports are being routed together.

In systems where many drives will be needed, SAS signals are often routed through backplanes instead of cables. This allows for “cleaner” systems and traditionally is more reliable and easier to maintain. It also helps to eliminate the confusion associated with multiple cabling options.

Once the components are all assembled, with connectors at the end ports, the system is now ready for SAS or Serial ATA (SATA) storage devices to be added to the system. The protocol included in the SAS host controllers as well as the SAS expander ports, makes it possible for OEMs, systems integrators and end-users to determine the right type of drive to be added into the system.

After the storage devices have been added into the mix, the system then needs to be tested to ensure that all of the various components are working in concert. Various hardware and software pieces are brought together to ensure that SAS is capable of running legacy SCSI middleware applications, that SATA devices can properly operate within the SAS infrastructure, and that the SAS topology can be scaled to support the promise of the new generations of SAS expanders.

The test and validation component truly brings all the pieces together and gives us the confidence that SAS solutions can deliver favorable end-user experiences.

Figure 1 describes graphically how all the various SAS components might be brought together. Other variations of this basic anatomy may also be viable.


Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.