A New Serial

The rapidly evolving data storage market has proved to be both a benefit as
well as a concern for enterprises. The constant drive to create competitive and
efficient products has meant lower TCO for enterprises. The challenge is then
keeping up with changing standards and ensuring interoperability and
compatibility with existing infrastructure. Serial Attached SCSI (SAS)
technology tries to alleviate some of these challenges by providing complete
backward compatibility while allowing scalability to next generations.

SAS is the logical evolution of the traditional SCSI interface, and it
represents the continuation of more than twenty years of technology development
and infrastructure investment. Initially, SAS was touted as a replacement
technology for perennial parallel SCSI interface but has now proven to be much
more than a performance connection for the standard high volume server market.

This data-transfer technology moves data to and from computer storage devices
such as hard drives and tape drives. Since the SAS interconnection enables the
attachment of either high performance SAS drives or high capacity desktop
oriented Serial Advanced Technology Attachment (SATA) drives, it offers the good
capability of servicing a wide range of enterprise storage solutions. These
include enterprise RAID storage systems, OLTP data warehousing/data mining;
large business servers, midline storage, nearline storage and tap libraries.

SAS depends on a point-to-point serial protocol that replaces the parallel
SCSI bus technology that first appeared in the mid 1980s in data centers and
work stations, and it uses the standard SCSI command set.

Why SAS?

The advantage of SAS technology lies in the way in which data moves. In a
serial transfer of data, bits of data move linearly in a single path.

In a parallel mode (traditional SCSI), multiple bits of data crowd the
pathway thereby affecting throughput (or performance of the existing
connection). The competition between bits of data that do not arrive
simultaneously leads to variable gating, buffer delays and varying signal path
lengths.

These problems do not occur in a serial transfer. Bandwidth utilization is
efficient in case of a serial data transfer and data integrity is preserved.
Serial Attached SCSI offers the possibility of data transfer at gigabit speeds.
It is cheaper than Fibre Channel and reliable than SATA. Serial Attached SCSI
enables faster device interconnect speeds, simpler cabling and improved system
reliability, while preserving existing SCSI capabilities.

The technology has received backing from technology industry companies such
as LSI Corporation, hard drive makers such as Fujitsu, Hitachi, and Seagate; and
system providers such as Dell, Fujitsu Siemens Computers, and NEC.

An IDC report on SAS observes that compared with historical interface
transitions, the transition to SAS technology has been relatively fast. This is
notable considering the timeline of other technologies. SCSI has been around for
over twenty-five years. Fibre Channel was introduced in 1996 and is still used
extensively within high performance external storage arrays within storage area
networks. SAS on the other hand, has largely replaced parallel SCSI solutions
for internal server storage.

Measuring Benefits

Serial Attached SCSI (SAS) has several attributes that can benefit vendors,
CIOs and users, system architects. SAS has features of SATA, SCSI, and Fiber
Channel technologies making it a storage device that can fit into most storage
environments and computing environments such as servers and workstations.

SAS can scale to more than 4,000 devices per port which means increased
opportunity for deployment in commercial and technical applications.

SAS also supports legacy SCSI software which means that migration to SAS is
cost-effective and less risky (from existing capital investment standpoint). It
also allows for backward compatibility to prior generations of SAS. An example
would be 3GB SAS (also known as SAS-1) is fully forward compatible with 6GB SAS
(also known as SAS-2).

Interoperability is a key feature of SAS that enables compatibility between
SAS products developed by different vendors thereby reducing the end-users
dependence on a given vendor.

SAS offers high availability of data due to the dual port capability in each
SAS drive. So, in the event of a controller failover, there is an extra path
available to each drive.

SCSI also improves connectivity to larger numbers of drives via a new type of
device called an Expander.

The technology allows increased device support and bandwidth scalability,
thereby reducing overheads that impact SCSI storage environment. SAS being a
serial standard allows for similar reliability advantages over parallel
standards such as SCSI. This comparison would be similar to PCI express
advantages over parallel PCI standards in host systems.

Trend Drivers

Security data privacy and regulatory requirements are pushing enterprise
storage demand. SAS supports variety of native security features and flexibility
to system architects.

Power and Cooling: Server and storage hardware cost is not the only factor
determining IT budgets in enterprises. Power and cooling are major areas of
concern too.The electricity cost of enterprises scale with the number of servers
and storage systems in those enterprises and the concentration of those systems.
Enterprises pay not only for the electricity to power the systems but also for
the electricity to cool the heat they generate.

Server and Storage Virtualization:

Virtualization enables a single server or storage system to behave as
multiple such systems. Treating one system as if it were many systems
effectively concentrates the performance and capacity needs of the former real
systems into that single system. Enterprise customers tend to increase the
storage and I/O requirements of such virtualized systems.

Evolution to 6GB: Enterprise CIOs will face their next business critical
technology decision as the Serial Attached SCSI (SAS) storage interface
transitions from the present day 3GB per second standard to the 6GB standard.

The first generation of the SAS interface, introduced in 2005, supports up to
3GB data rate performance. It accelerated the performance capabilities of
parallel SCSI by supporting full-duplex data transmission and multiple
simultaneous point-to-point connections while maintaining compatibility with the
SCSI protocol. Supporters of the SAS ecosystem, including OEMs, board suppliers,
and semiconductor suppliers, are now preparing 6GbB SAS products that will be
backward compatible with existing 1.5GB SATA and 3G /s SAS/SATA products and
infrastructure.

While 6GbB SAS is a performance improvement over 3GbB SAS, it also offers
some new benefits for enterprise applications, including more storage capacity
per given server; improved overall system performance by distributing the
servers I/O requests across a greater number of HDDs and better rack-to-rack
interconnect signal reliability enabled by a new feature called decision
feedback equalization (DFE).

IDC believes that applications that will lead the transition to 6GB SAS will
be those that can benefit from the additional throughput provided by the
technology. These applications include virtualized servers, high-end servers,
backup and video streaming. IDC also expects the most likely timing for 6Gb/s
SAS to be OEM server production volume in late 2009, followed by widespread
market adoption by early to the middle of 2010.

With companies like Dell, Fujitsu Siemens Computers, IBM, Intel, NEC and Sun
Microsystems choosing 6GB SAS RAID-On-Chip (ROC) solution to enable next
generation entry to enterprise server platforms, the 6GB evolution is well on
track to full fledged adoption. This will continue to pave way for SAS
penetration into high reliability storage area network focused applications
(traditionally dominated by Fiber Channel technology) compared to traditional
solutions.

Pravin Desale

The author is the managing director of LSI Corporation, India

maildqindia@cybermedia.co.in