Although serial attached small computer system interface
(SCSI) disc drives and interfaces are not yet a reality, five
manufacturers - Adaptec, Fujitsu, Hitachi, Maxtor and Seagate -
have announced they will ensure their serial attached SCSI products
will be interoperable.
SCSI is an electronic computer standard that allows computers to
communicate with peripheral devices such as disc drives, CD-Rom
drives, printers, tape drives and scanners. SCSI interfaces are
currently deployed in a parallel fashion, but this new industry
initiative is aims to deploy SCSI using a serial-attached
format.
Linus Wong, director of strategic marketing, storage solutions
division at Adaptec, said serial-attached SCSI is a natural
evolution from its currently deployed counterpart, parallel SCSI,
and will be on the market in about 12 months.
"SCSI has doubled in performance every couple of years in the
past five to 10 years. We are on our seventh generation of SCSI at
320 megabits per second," Wong explained. "But each of those
transitions have got more and more difficult. Serial technologies
in general allow you to break down that barrier and make a leap to
that next generation technology."
Wong also sits on the board of the industry body the SCSI Trade
Association, which drives positioning and standards around SCSI. He
said the association has abandoned plans for what would have been
the next generation of parallel SCSI - Ultra-640 - and is now
focusing on serial-attached SCSI which would initially reach speeds
of 1.5gbps.
"In the parallel technology you’re sending multiple bits down
the wire at the same time, and they all have to get there at the
same time and be clocked at the same time. So as you go faster it
becomes almost impossible to get all those bits to line up," Wong
explained.
"At the current generation of SCSI there are six bits on the
wire at the same time. So to get those to line up and arrive at the
other side of the cable at exactly at the same time is very
difficult."
But with serial-attached SCSI one bit is sent at a time, so
there is no need to worry about clocking or skew compensation. That
is why Wong says these faster speeds can be attained. In addition
to the higher bandwidth, serial-attached SCSI devices will also be
able to transmit data over large geographic distances.
Parallel SCSI can only address a limited number of devices and
has performance as well as distance limitations plus large
connectors that make it unsuitable for some dense computing
environments.
Wong says serial-attached SCSI disc drives can be attached to
many devices, while parallel SCSI drives, because they share a bus,
can only be attached to about 15 devices.
"Because [serial-attached SCSI] is a point-to-point technology,
each device has a dedicated link, you can fan out to many more
devices," Wong said. "So, from a spec point of view it is tens of
thousands, but for all practical purpose the limit is probably 120
to 256 devices."
One drawback is there will be no compatibility with
serial-attached SCSI devices to parallel SCSI drives. However, Wong
said bridges will be included that can span serial-attached SCSI
and parallel SCSI environments when the technology hits the
streets.
This means a user would have to invest in new hardware to make
the transition from parallel to serial-attached SCSI, but Wong said
the software needed to administer these two environments would be
the same. Only minor adjustments in modifying parallel SCSI
protocols to serial-attached SCSI protocols would be required.
In addition, from a standards perspective, Wong said the T10
Technical Committee, part of the InterNational Committee on
Information Technology Standards, which has defined standards for
parallel SCSI, is also working on developing standards for
serial-attached SCSI.
Another benefit, Wong said, is that serial-attached SCSI
interface supports serial-attached SCSI disc drives and Serial
Advanced Technology Attachment (Sata) disc drives.
Like serial-attached SCSI, Sata is a relatively new technology.
Wong says the first products are just starting to ship now and is
slated to replace parallel ATA disc drives such as integrated drive
electronics (IDE), enhanced IDE (EIDE) and UDMA 66, usually seen on
the desktop environment.
With Sata, users can get faster speeds than with parallel ATA,
and, enterprises are showing interest in deploying them in a server
environment in lieu of SCSI drives for storing reference data
because Advanced Technology Attachment drives are less
expensive.
Unlike transactional data, which is constantly being accessed
and requires updating in real-time, reference data does not change
and is not accessed often so, Wong said, enterprises do not mind
compromising speed for cost-savings in this instance.
While Advanced Technology Attachment drives do not perform as
well as SCSI drives, Wong said this could be partially compensated
for by Redundant Array of Independent Discs (Raid) software and
Sata disc drives are ideal for storing reference data.
"[The industry is] positioning those as an alternative to tape
drives, or at least a staging between tape and disc because it is
less expensive," said Alan Freedman, research manager,
infrastructure hardware with IDC Canada.
"You don’t get quite the performance [as SCSI] although I don’t
think there is a huge decrease anymore. But, especially for things
like fixed content which is going to change a lot and which you
don’t need to recall all that often, this is a good option."
Wong said the interoperability of serial-attached SCSI
interfaces with serial-attached SCSI disc drives and Sata disc
drives would mean enterprises, OEMs and resellers could be flexible
in their storage deployments.
Freedman said this is yet another option for people to
prioritise their data storage needs.
"It gives them the chance to create a storage infrastructure
that is properly aligned with their business initiatives and that
enables them to save money while getting the performance that is
necessary," Freedman said. "It is not always about getting top
performance, it is about getting the performance that you
need."