SAS Means Freedom of Choice

Author: Chris Croteau
Intel Corporation

undergoing an exciting serial transformation, one that goes
far beyond a mere parallel-to-serial conversion to deliver
significantly increased capabilities. Serial Attached SCSI
(SAS) is the next evolution of SCSI technology following Ultra320
SCSI, utilizing a speedy serial point-to-point architecture
to eliminate the bus overhead of today’s parallel SCSI
technology. Dual-ported devices are also a standard feature
for Serial Attached SCSI, enabling multiple signal paths for
each disk drive. In addition, Serial Attached SCSI will have
the ability to scale to over 16,000 physical devices in a
single domain through the use of expanders. And SAS leverages
existing SCSI protocols, thus ensuring backward compatibility
with legacy SCSI drivers and enterprise application software.

In addition
to compatibility with existing SCSI drivers and software,
Serial Attached SCSI offers interoperability with Serial ATA
devices. This is an unprecedented development in storage technologies.
The inclusion of the Serial ATA Tunneling Protocol (STP) allows
Serial Attached SCSI environments to host Serial ATA devices.
This will not only allow flexibility in system deployment,
but also offers end users more choices based upon economics
and workloads for specific applications.

To take
advantage of this SATA protocol compatibility within Serial
Attached SCSI, interoperability of connectors and backplanes
was also required. Thus SAS and SATA connectors were designed
to be fully compatible when used in SAS environments. While
Serial ATA is currently only single-ended, Serial Attached
SCSI can host dual ports for path redundancy. To ensure proper
functionality, SAS drives lack the notch found on Serial ATA
drive connectors and thus cannot be inserted into Serial ATA-only
enclosures. This precaution was necessary because SATA host
controllers don’t understand SCSI protocols and hence
won’t recognize a SAS drive.

Why is
SAS/SATA compatibility a great step forward for enterprise
storage? With the delivery of Serial Attached SCSI and Serial
ATA products, end users will finally have the power to choose
the best solutions to meet their needs. Backplanes, disk drives
and RAID controllers can all be customized for maximum return
on infrastructure investment. Reference data (e.g., archived
records, digital images and media) can be warehoused using
lower cost Serial ATA drives, while transactional data (i.e.,
stored data that is in high, real-time demand) can be stored
on higher-performance Serial Attached SCSI drives. Thanks
to the compatibility of SAS and SATA, both drive types can
be housed in a single SAS backplane to create an economical,
powerful array at a fraction of the cost of more complex storage
technologies currently in use.

managers strive for more comprehensive management of their
data throughout its lifecycle (known as Information Lifecycle
Management or ILM), they’ll need platforms of differing
costs and capabilities as they move data off of mainstream
storage arrays and onto lower cost near-line arrays. SAS brings
new performance and scalability to today’s mainstream
SCSI applications while also providing the capability to house
lower cost SATA drives for near-line platforms. Regardless
of which device they choose, SAS gives IT managers the power
and flexibility to make their own decisions on disk drives.

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