Flexibility for the Future

Author: Tonya Comer

server solutions lack flexibility in the type of storage they
deploy. Systems designers for direct attached storage (DAS)
must choose between a low cost storage interconnect like Serial
ATA, a mainstream interconnect like U320 SCSI or a high-end
interconnect like Fibre Channel. The interface decision dictates
the remaining storage infrastructure costs, limiting users’
ability to deploy and redeploy core technologies, and potentially
adding significant expense and management burden to the enterprise.

Attached SCSI (SAS) solves this flexibility problem by utilizing
a single interface that provides customers with a choice of
storage devices. By designing in compatibility with Serial
ATA (SATA) drives at the connector, SAS allows a low cost
alternative to the higher reliability SAS disk drives. SAS
can communicate with the existing 1.5-Gb/s data rate SATA
drives and extends the high-speed serial signaling by introducing
3-Gb/s data rate SAS drives with a roadmap to 12-Gb/s and

SAS technology
leverages the backplane connector introduced by SATA and extends
it by adding support for a second communication link for dual-ported
SAS drives. SAS also improves the signal characteristics of
the physical components to allow for greater length cables
with increased reliability. By contrast, desktop SATA controllers
and drives are designed with cost- and power-constrained physical
link components, limiting cable length and increasing the
potential for link errors in SATA backplane solutions. To
minimize link errors when SATA drives are connected to SAS
devices, SAS expanders and controllers are designed to compensate
for the limitations of SATA drives.

SAS drives can fully support multi-initiator access, and SCSI
Tunneling Protocol (STP) enhances SATA disk drives by enabling
a limited form of multi-initiator access. As a result, vendors
can use the same Stock Keeping Unit (SKU) for the backplane
to support both SAS and SATA drives. This reduces interoperability
issues and inventory costs and simplifies product support

By virtue
of its multi-protocol support, a SAS-based storage solution
will enable customers to deploy a standardized server interface
and choose the most appropriate storage device for their application.
Low-cost SATA disks currently being used in desktop solutions
will be available for less demanding applications, while enterprise-class
SAS disks (leveraging industry-leading SCSI specifications)
will provide the continuity, reliability and performance demanded
in mainstream server applications. In addition, smaller form
factors and greater addressability will lead to a new level
of flexibility when deploying mainstream data center servers
and subsystems.

SAS leverages
the SATA interconnect technology to provide:

  • simplified
    cable, connector, and backplane design
  • compatibility with lower-cost drives

SAS extends the SATA interconnect technology to provide:

  • increased device support and bandwidth scalability
  • support for multi-port and multi-path connections
  • support for topology and link management
  • support for enclosure management
  • support for the design of new small form factor hard disk drives
  • improved
    backplane support




compatibility has widespread benefits for both the industry
and enterprise users. For OEMs and integrators, one of the
primary gains from compatibility between SAS systems and SATA
disk drives will be the ability to have a common infrastructure
of cables, connectors, backplanes, cabinets, etc. VARs and
integrators can focus on matching the processing and storage
needs of the customer, with fewer compromises because of storage
interface limitations—no need to worry about installing
the proper backplane and cables.

<End users
will welcome lower prices resulting from the cost reductions
that SAS/SATA compatibility provides to backplane manufacturers,
system OEMs and VARs. In addition, the ability to change from
SATA to SAS drives without purchasing new systems will greatly
simplify the upgrade process, and help future-proof end user

SAS was
developed to meet the changing demands of mainstream enterprise-class
storage system customers. By accommodating both low-cost bulk
storage needs (SATA devices) and the performance and reliability
demands of mission-critical applications (SAS devices), SAS
minimizes customer investment while maximizing customer choice
and ease of deployment. As storage requirements evolve customers
can easily adjust their solutions to match, without a ‘fork-lift’
upgrade. SAS provides flexibility for the future, protecting
enterprise users who will eventually need more power, easier
connectivity, and greater scalability as storage requirements
continue to escalate.


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