Serial Hard Drive Interface Compatibility

Jim Pascoe
Hitachi Global Storage Technologies

Use of
the parallel bus interface, which has long been the industry-standard
storage interconnect, is on the verge of decline among the
vast majority of system builders. Components that are based
on the aging interface are increasingly being replaced by
their evolved serial counterparts—Serial Attached SCSI
(SAS) and Serial ATA (SATA).

The new
serial interfaces are designed to address the bandwidth limitations
of their predecessors. They will support faster data transfer
rates and more devices per controller, as well as reduce the
size and complexity of the cables and connectors (thus enabling
smaller, more densely-packed disk arrays). Among the most
significant benefits is that incompatibility between SCSI
and ATA will finally be addressed.

For the first time, system builders will be able to easily integrate
SAS and SATA hard drives in a single enclosure. Plug compatibility
is one reason for this, but a more substantial explanation
lies with the system controllers themselves. The controllers
have been designed to recognize both interface types and “talk”
to each device in its own language and coordinate their respective
activities and performance.

As a result
systems can now be deployed using a combination of SAS drives
(to provide the highest levels of system reliability) and
SATA drives (to provide a high performance and low-cost-per-gigabyte
solution for near-line storage, disk-to-disk backup and similar
applications). This allows for a great deal of flexibility
in designing disk arrays and other storage systems that provide
exactly what the application requires for the least cost and
maximum return on investment.

SAS- and SATA-based hard drives can now coexist within a system,
it’s important to understand each of their respective
attributes and intended environments. The following is a brief
overview of each serial interface and a set of recommendations
for selecting a storage solution that is geared to meet your
specific needs.

Attached SCSI
SAS will continue to build upon the established SCSI
technologies that are typically used in RAID and other enterprise
environments. The emerging interface uses the existing SCSI
protocol, while featuring serial point-to-point interconnections,
dual porting, increased addressability and full duplex operation.
SAS is set to deliver a maximum data transfer rate of 3Gbit/s,
with a roadmap to 12Gbit/s. SAS will also simplify the design
process for system builders because it uses simplified cable
routing and shares similar physical attributes and interface
transfer rates with SATA. The SAS feature set will enable
the development of high performance and reliable solutions
that can be deployed quickly and easily.

SATA is an interface technology that was engineered to
be the Parallel ATA replacement for PCs, workstations, and
other ATA applications. Additional features have been developed
into SATA to enhance enterprise ATA applications. SATA currently
delivers a maximum data transfer rate of 1.5Gbit/s per second,
and will have future speeds of 3Gbit/s followed by 6Gbit/s.
The interface has been designed to simplify device configuration
and improve interface level data integrity. Like the SAS interface,
SATA disk drives use a compact cabling structure and smaller
connectors, which enable enhanced air flow and reduce system

the Right Hard Drive
There are several factors to consider when selecting
the right disk drive/interface for a given application. There
are substantial differences between desktop and enterprise-class
drives in terms of capacity, performance, reliability, scalability
and cost. SATA drives are most commonly used in desktop applications
because of their low cost-per-megabyte, extremely large capacities
and performance features. Here, the focus is single-user applications.
In some cases, SATA drives can be used in enterprise “entry-server”
and other data-intensive applications.

SAS hard
drives will be used in enterprise environments (e.g., server
storage, high-end workstations), where multiple users are
accessing a single system. These mission-critical installations
will require the highest levels of system performance for
applications such as online transaction processing, data analysis
and storage virtualization.

between the new serial interfaces benefits system builders
and end users alike. The common serial interconnect will enable
system builders to deploy systems that share common backplanes,
connectors and cabling. This greatly simplifies the process
of changing the “mix” of drives within an enclosure,
enabling users to easily replace a SATA drive with a SAS drive
if their needs change.

End users
will have additional flexibility in buying systems that are
optimally configured to address their target application.
The ability to replace a SATA drive with a SAS drive also
eliminates the need to replace entire systems when additional
performance/reliability is required. These factors combine
to substantially reduce the total cost of ownership for desktop
and enterprise storage.

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