The Many Faces of SAS – Beyond the DAS Factor

Author: Greg Schulz, Founder
StorageIO Group

Serial Attached SCSI (SAS) as a technology, has several meanings similar to the acronym SAS in general, which for IT professionals also refers to the Statistical Analysis Software from the SAS Institute. There is a linkage between SAS as a software package and SAS as a storage technology, particularly for performance and capacity-planning analysts that forecast storage usage and performance. However for now, let’s focus on SAS as a storage interface technology, leaving the discussion about performance and capacity planning for another time.


Figure 1: SAS in block (DAS and SAN) along with file (NAS) storage systems


The reason I bring up the two meanings of SAS (as an acronym), is that SAS as a storage interface technology also has multiple personalities and capabilities as seen in Figure 1. To be more precise, SAS can, and is being used today as a means of attaching storage devices to servers as well as a means of attaching hard disk drives (HDDs) to storage systems and their controllers. A common question I hear in the market place from IT customers and solution providers is – when will vendors start shipping SAS products and the answer is a qualified right now!

A qualified yes for SAS-based products shipping now is because SAS ecosystems are broad-ranging from protocol interface chips, interposer and multiplexers to enable dual-porting of SATA disks to SAS interfaces, SAS expanders, SAS host-based PCI-X and PCIe, as well as external RAID controllers and 3.5″ and 2.5″ SAS HDDs, among other components. SAS HDDs are also being deployed inside storage systems (See figure 2) across different price bands and market segments for both block-based as well as NAS file-based storage solutions. Vendors’ SAS-based product offerings will vary based on their target market or specific-solution target-price band for deployment with components first, followed by adapters and entry-level block and NAS storage systems, followed by enterprise class solutions.


Figure 2: The many faces and deployment opportunities for SAS HDDs


SAS HDDs are either replacing or serving as an alternative for Fibre Channel (FC), Parallel SCSI and in some cases Serial ATA (SATA) disks, while in others, co-existing with SATA devices. In addition to being used as a means of attaching HDDs to servers and storage systems controllers, SAS is also being used today to attach storage systems, for example mid-range and entry-level storage arrays, to servers as an alternative to FC, Parallel SCSI or iSCSI for open-systems block-storage access.

Four Common SAS Storage-Based Technology Deployment Scenarios Include:

Dedicated direct attached storage – DAS internal
SAS HDD on a server blade, SAS HDD in traditional server for boot or local data replacing parallel SCSI or SATA HDD, dedicated HDD in a NAS appliance either as primary and expansion storage, or as primary storage with expansion using FC- or SATA-based HDD.

Dedicated direct-attached storage – DAS external
SAS HDD, or mix of SAS and SATA HDDs, configured in a storage enclosure (JBOD) or RAID array attached via SAS to a single server using a SAS host controller or SAS host RAID adapter. The storage may be served via the network file system (NFS) or common internet file system (CIFS), aka windows file sharing, to other servers and clients to support NAS.

Shared DAS – External
SAS HDD on a storage blade in a blade center, SAS HDD or mix of SAS and SATA HDDs in a storage or RAID array or storage appliance that attaches to two or more servers via SAS.

Some direct-attached storage products offer dual-controller active/active support, built in performance analysis and management, including drive rebuild. These products can often mix SATA and SAS disk drives for storage tiering. One system at National Association of Broadcasters supported as many as 60 drives although it is being expanded to support up to 96 drives. Storage systems with capacity up to many tens of TBs are currently on the market, but with 1 TB drives ramping up (for SATA at least) these storage capacities will swell within the next year.

Shared SAN or NAS external storage
SAS HDDs attached to servers via iSCSI, FC or even InfiniBand SCSI remote protocol (for SCSI on InfiniBand) (SRP) for block access and NAS (NFS or CIFS) over Ethernet or InfiniBand IP-based networks for file-based data and storage sharing access. Some storage systems support multiple concurrent storage protocols for attachment to servers that can include some combination of iSCSI or NAS using Gb Ethernet, SRP or NAS via IP on InfiniBand, SAS and FC including FC-SB2 (aka mainframe storage access over FC) (FICON) as well as SCSI/FC Protocol (aka SCSI on FC) (FCP). Similarly some storage systems support SAS as a primary HDD in the base storage system with some vendors supporting SAS-based expansion modules, while other vendors support FC and FC-SATA based expansion modules. For example, just because a storage system has SAS-based HDDs in the main storage system may not mean that expansion drives are also SAS-based, do your homework to avoid surprises.

What makes SAS appealing as both a server-to-storage, and storage-controller-to-HDD interconnect for DAS, SAN and NAS, as well as storage appliances such as virtual tape libraries (VTLs etc.) include:

  • Availability and resiliency of enterprise class HDDs in 2.5″ high-performance small form factor energy efficient and 3.5″ high-capacity disk drives with native dual-port support along with other HDD characteristics found in FC and parallel SCSI enterprise disk drives.
  • Performance including 3Gb/sec (with 6Gb/sec in development and 12Gb/sec on the future roadmap) per link bandwidth (send and receive) for narrow, with up to 4 links supported per wide physical SAS port, enabling an aggregate port bandwidth of 12Gb/sec (4 x 3Gb/sec).
  • Connectivity to support multiple devices and links per physical SAS port, along with supporting more SAS devices at longer and faster distances than parallel SCSI.
  • Flexibility, including concurrent support for attachment (of both SAS and SATA devices) to a SAS port for co-existence eliminating the need for FC-SATA components and associated support technology to deploy tiered disk drives in a storage system.
  • Cost saving opportunities for manufactures, eliminating the need for extra power-consuming components (as well as field-based spare component kits), component costs and sizes compared with supporting separate FC-SATA technologies.

General tips and recommendations include (among others) doing your homework, discussing with your solution providers and manufactures their current SAS-enabled technologies as well as what they are planning to deliver in the not so distant future – under NDA if applicable. Small form factor (SFF) 2.5″ enterprise class disk drives should not be confused with 2.5″ notebook and mobile disk drives that have different service characteristics. The increased density per enclosure of 2.5″ SFF disk drives means more potential IOPs and capacity per cubic foot, however it also means more weight, power and heat to be removed per cubic foot.

Look at how vendors will roll out SAS-based and -enabled storage solutions and how those deployments co-exist and interact with existing technologies. Investigate how expansion storage is added to NAS and block-based storage systems (for example), does the storage controller use FC to attach external storage expansion modules with FC, SAS or SATA HDDs, or does the storage controller support SAS-based back-end disk expansion ports? Understand the benefits and caveats today and into the future of mixing SAS and SATA HDDs in the same disk enclosure shelve and attached to the same SAS controller port. Similar to how FC-based disk drives replaced parallel SCSI HDDs in primary storage systems, SAS is well positioned as a successor to FC disk drives, initially in entry-level and in the future high-end enterprise class storage systems. From a performance perspective, benchmark comparisons exist on the Storage Performance Council (SPC) website for block-based storage systems at

To wrap up, if you are involved with storage performance and capacity planning, add SAS in the context of Serial Attached SCSI to your vocabulary as well as you update your SAS (as in SAS software) models and lookup tables to include the performance and service characteristics of SAS-based storage systems. There are many faces to SAS-based storage technology that has breadth, maturity and reach from dedicated, internal direct-attach storage replacing parallel SCSI in servers, to an interface on SAS HDDs that can support enterprise-class deployment scenarios. Learn more about SAS related topics, technologies and components including technical tips at the SCSI trade association website ( ) and the StorageIO Group web site (

About the author
Greg Schulz is founder and senior analyst of the StorageIO Group and author of the book Resilient Storage Networks — Designing Flexible Scalable Data Infrastructures (Elsevier Digital Press). StorageIO’s services include content generation (white paper, articles), speaking engagements (seminars, web casts, customer events) and advisory (product and technology positioning) among others. With more than 25 years’ experience in the IT and storage industry, Greg has held strategic positions in management, marketing, sales, technology development, and data center management. Prior to working as an industry analyst and vendor, Greg worked in IT organizations at various firms including an electric power utility, railroad and benefits processing firm among others. At StorageIO, Greg consults on technology strategy and positioning to tier-1 through emerging startup vendors as well as IT organizations of all size on storage, networking and management software matters.

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