SATA Drives: Redefining Reliable Storage

Today a hard drive data transfer rates run at about 30 times faster as
compared to what they did 15 years ago, and this has redefined the way we have
been storing the data. Traditional interface, Parallel ATA (PATA) has been in
use for about 15 years and now has been replaced by Serial ATA drives (SATA).
According to a research done by Gartner Group in April 2003, close to 90% of ATA
drives would be 150 MB/s SATA drives in 2006, with 300 MB/s drives making a
sneaky entrance. By 2007 almost 80% of SATA drives will be on 300 MB/s.

PATA was a relatively simple, high performance, and low cost technical
platform, it had reached its limits for both performance and reliability when
data transfer speeds significantly exceeded 100 Mbytes per second. PATA's
success has been attributed in the past to frequent improvements in the
interconnect's speed and overall performance. Overtaking this platform is
Serial ATA (SATA) drives, that came in existence with a set of evolutionary
improvements of ATA. SATA can come in various capacity points, majorly in four
capacities 80, 120, 160 and 200 GB. Each drive model comes with an 8 MB cache, a
boost from the previous industry standard of 2 MB cache. Let us look at the
benefits of SATA and indirectly understand how it will replace PATA after its 15
years of service to the PC users.

WHAT DRIVES SATA's SUCCESS?

With various factors driving the storage landscape, SATA has become the most
viable storage platform. It has consistent high performance with cost-effective
pricing. Its external transfer rates are about 50% faster than the Ultra ATA/100
bus, most commonly found today in desktop PCs. SATA can give desktop drives at
least another 10 years of performance growth from 150 MBPS in its first
generation and upto 600 MBPS in its third generation.

Scalability is also an issue that defines the TCO for the CTO. Today's SATA
components are intended to provide greater flexibility to system designers in
many ways. With an ease of use the administrator need not struggle with the
40-pin bulky cable anymore. The SATA has thinner and up to 1 meter in length
cables that allow for creative form-factor designs and improved airflow dynamics
internally within the PC.

Another reason that has put SATA ahead of PATA is reliability of data,
command and status management. SATA cables and connectors were developed to
maintain very high data integrity, reducing data crosstalk even with higher
speeds. Cyclic Redundancy Checking (or CRC) is used to ensure the data, commands
and status components of your data transmission are received correctly. Previous
CRC implementation of ATA protocol verified the data component only.

SATA, unlike PATA, is a point-to-point architecture. This means that each
device connects directly to the host via a dedicated link. Each device owns the
entire bandwidth, i.e. 150 Mbytes per second, rather than sharing it with
another drive and causing possible performance bottlenecks. This also eliminates
the often-cumbersome master/slave delegation. With intelligence on each link,
SATA drives will offer an advantage in a RAID (Redundant Array of Independent
Disks) environment.

THE FEATURE FACET

Today, native Serial ATA has gone into overdrive with features that will
truly exhibit its superiority in the interface domain. Now some SATA drives
comes embedded with proprietary features that are redefining the simple data
transfer functions. A feature such as NCQ response to the workloads created by
Microsoft Windows XP as well as Linux systems. Both these environments make use
of multi-threaded software or processor-based, hyper-threading technologyboth
features that can result in multiple drive commands being outstanding at the
same time. A need arises to reorder outstanding commands in order to improve I/O
latency and reduce mechanical wear and tear on drives.

With NCQ, drive performance can increase significantly for such diverse
workloads. NCQ is a command protocol in Serial ATA that allows multiple commands
to be outstanding within a drive at the same time. SATA drives that support NCQ
have an internal queue in which outstanding commands can be re-ordered or
dynamically rescheduled. NCQ also provides the critical tracking mechanism for
outstanding and completed portions of the workload. In addition, NCQ includes a
mechanism that makes it possible for the host to issue additional commands while
the drive is seeking data for another command. NCQ, in simple terms, functions
much like an elevator in a high-rise building. The elevator does not pick up and
let out riders in the order they push the floor request buttons. That would mean
excessive wait times and speedy breakdown of the elevator. Instead, the elevator
follows a sequential order, traveling up and down from floor to floor.

Another major technical feature that is considered important in today's
world is the power management, and there are various methods to do so. One such
method is Deferred Spinup. It allows the storage subsystem controllers to
sequence the disc drive initialization process. In systems with multiple Serial
ATA drives, Deferred Spinup lets the subsystem controllers stagger the spin-up
of each drive and more efficiently accommodate the available power supply
current.

There are newer SATA connectors and cables designed by various vendors that
provide additional enhancements. The new connectors now include a latch for
increased retention; the latch also gives tactile feedback to a user connecting
a drive without seeing the connection point.

THE FUTURE

The present implementation of SATA drives is via PCI-based SATA Cards and
on-board motherboards. ICH5 boards typically carry 2-SATA ports and 2-PATA
ports. However, with ICH6 expected to make an appearance in July 2004, that can
give 4-ports and 1-port SATA and PATA implementations respectively. With
4-ports, setting up RAID-5 configuration would give users the performance and
reliability they expect from SATA.

Hot-Pluggability is also an issue that is being addressed, especially for the
SMB segment. Two basic mechanisms for hot-plug are 'Receptacle/Backplane
Pre-charged' and 'Presence Detection Circuitry' As new markets develop
uses for low-end, near-line storage servers with an ATA backplane, SATA
connectors that are designed with hot plugging and show hot swapping capability,
help eliminate the need for spaghetti of cables.

SATA ADOPTERS

SATA technology is adopted by the users of entry servers, such as Telecom
servers, Messaging servers, Collaboration servers, Engineering servers,
Scientific and R&D servers and Workstations. There are various verticals
that are the heavy users of near-line storage and bulk storage, or engage in
high-volume data collection. These include Wireless carriers, Telephony
providers, Healthcare providers, Customer service organizations, Manufacturing
plants, Pharmaceutical research, Universities and educational institutions,
National defense and other governmental agencies.

Sharad Srivastava
is Country Manager-India, Seagate Technology.