For over 20 years, the parallel bus interface has been the mainstream storage
interconnect for most storage systems. But increasing bandwidth and flexibility
demands have exposed inefficiencies in the two main parallel interface technologies:
SCSI and ATA. The lack of compatibility between parallel ATA and SCSI - including
different connectors, cables and software - increases costs for inventory management,
R&D, training and product qualification.
While parallel technologies meet most performance requirements of today's
enterprise solutions, continued demands for higher speeds, more robust data integrity,
smaller designs and wider standardization cast doubt on the ability of parallel
technology to economically keep pace with increasing CPU processing power and
disk drive speeds. In addition, shrinking budgets are making it increasingly difficult
to sustain the costs of developing and managing multiple backplane types, validating
multiple interfaces and stocking multiple I/O connections.
Parallel technology poses still other challenges. Parallel transmissions are
susceptible to crosstalk across wide ribbon cable paths. This crosstalk adds line
noise and can cause signal errors, a pitfall that has been remedied by slowing
the signal, limiting cable length or both. Terminating parallel signals is also
difficult, requiring individual lines to be terminated, usually by the last drive,
to avoid signal reflection at the end of a cable. Finally, parallel's large cable
and connector size make it unsuitable for increasingly dense computing environments.
Introducing SAS and SATA
Serial technology, specifically Serial ATA (SATA) and Serial Attached SCSI
(SAS), addresses the architectural limitations of its parallel counterparts. The
technology draws its name from the way it transmits signals, that is, in a single
stream (serially) compared with the multiple streams found in parallel technology.
The main advantage of serial technology is that while it moves data in a single
stream, it does so much faster than parallel technology because it is not tied
to a particular clock speed. Serial technology wraps many bits of data into packets
and then transfers the packets up to 30 times faster than parallel down the wire
to or from the host.
SATA extends the ATA technology roadmap by delivering disk interconnect speeds
starting at 1.5G bps. Due to its lower cost per gigabyte, SATA will continue as
the prevalent disk interface technology in desktop PCs, sub-entry servers and
networked storage systems where cost is a primary concern.
SAS, the successor technology to the parallel SCSI interface, leverages proven
SCSI functionality and promises to greatly build on the existing capabilities
of the enterprise storage connection. SAS offers many features not found in today's
mainstream storage solutions. These include drive addressability of up to 16,256
devices per port and reliable point-to-point serial connections at speeds of up
to 3G bps.
In addition, due to its small connector, SAS offers full dual-ported connections
on 3.5-in. and smaller 2.5-in. hard disk drives, a feature previously found only
on larger 3.5-in. Fibre Channel disk drives. This is an essential feature in applications
requiring redundant drive spindles in a dense server form factor such as blade
SAS improves drive addressability and connectivity using an expander that enables
one or more SAS host controllers to connect to a large number of drives. Each
expander allows connectivity to 128 physical links, which may include other host
connections, other SAS expanders or hard disks. This highly scalable connection
scheme enables enterprise-level topologies that easily support multi-node clustering
for automatic failover availability or load balancing.
In one of its most significant advances, the SAS interface will also be compatible
with lower-cost-per-gigabyte SATA drives, giving system builders the flexibility
to integrate either SAS or SATA devices while slashing the costs associated with
supporting two separate interfaces. As the next generation of SCSI, SAS bridges
the parallel technology gap in performance, scalability and affordability.
Multiple layers of compatibility
- Physical layers
The SAS connector is a universal interconnection that is form-factor compatible
with SATA. It allows SAS or SATA drives to plug directly into a SAS environment
for mission-critical applications with high-availability and high-performance
requirements or lower-cost-per-gigabyte applications such as near-box storage.
SATA connector signals are a subset of SAS signals that enable the compatibility
of SATA devices and SAS controllers. SAS drives will not operate on a SATA controller
and are keyed to prevent any chance of plugging them in incorrectly.
In addition, the similar SAS and SATA physical interfaces enable a new universal
SAS backplane that provides connectivity to both SAS drives and SATA drives. This
eliminates the need for separate SCSI and ATA drive backplanes. This consolidation
of designs greatly benefits both backplane manufacturers and end users by reducing
inventory and design costs.
- Protocol layer
SAS consists of three types of protocols, each of which is used to transfer different
types of data over the serial interface, depending on which device is being accessed.
Serial SCSI Protocol (SSP) transfers SCSI commands, and SCSI Management Protocol
(SMP) sends management information to expanders. Meanwhile, SATA Tunneled Protocol
(STP) creates a connection that allows transmission of the SATA commands. By including
all three of these protocols, SAS provides seamless compatibility with today's
existing SCSI applications, management software and SATA devices.
This multi-protocol architecture support, coupled with the compatibility of SAS
and SATA's physical connection, allows SAS to operate as the universal interconnection
for both SATA and SAS devices.
Benefits of compatibility
SAS and SATA compatibility offer a number of benefits to system builders, integrators
and end users.
System builders can now leverage the universal SAS/SATA connection to deploy
common backplanes and common connector and cabling devices. Upgrading from SATA
to SAS is as simple as replacing the disk drives. With parallel technologies,
upgrading from ATA to SCSI means replacing backplanes, connectors, cables and
drives. Other cost-saving benefits of compatibility include simpler validation
and inventory management.
VARs and system integrators will be able to easily configure custom systems
by simply installing the appropriate disk drive. This is because working with
dissimilar technologies and using specialized connectors and different cabling
will no longer be necessary. Moreover, the added price/performance flexibility
will better enable VARs and system integrators to differentiate their products.
For end users, SAS and SATA compatibility offers a new level of price/performance
flexibility. SATA drives will suit those requiring the best price advantage for
servers and storage deployments, while SAS drives will deliver the highest performance,
reliability and software management compatibility. The ability to upgrade from
SATA to SAS drives without having to buy a new system will greatly simplify the
purchasing decision, future-proof system investment and reduce the total cost
On January 20, 2003, the SCSI Trade Association (STA) and the Serial ATA (SATA)
II Working Group announced a partnership to enable SAS system-level compatibility
with SATA hard disk drives.
This collaboration, as well as cooperation among storage vendors and standards
committees, will further define compatibility guidelines - a move that will help
system builders, IT professionals and end users more finely tune their systems
to optimize application performance and reliability and reduce total cost of ownership.
The SATA specification reached revision 1.0 in 2001, and SATA products are
available today from a variety of manufactures. The SAS specification revision
1.0 is targeted for release in early 2003, with product availability in the first
half of 2004.
About the author
Paul Griffith is strategic marketing manager for Future I/O Technologies at
Adaptec, Inc. For more information, please visit: http://www.scsita.org,
By Paul Griffith
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