PMC-Sierra recently had the opportunity to interview Tim Symons,
Storage Architect & Technologist in the Storage Solutions
Group at Adaptec, to discuss Serial Attached SCSI (SAS) and
how customers will make the transition from parallel SCSI
to SAS. Adaptec offers direct-attached and networked data
storage for mission-critical applications, from data warehousing
and transaction processing to streaming video, medical imaging,
digital pre-press and more.

With
the new introduction of Serial Attached SCSI, what applications
will remain SCSI and which applications will migrate to SAS?
I believe that high bandwidth and high IOPS-oriented
applications will be the ones to migrate to SAS first. Large
disk arrays will also be quick to adopt SAS due to the simplified
management of 128 drives per segment and support for 128 segments,
as opposed to 14 drives in SCSI.

As
customers migrate to SAS, will they be able to preserve any
of their existing investment in SCSI?
SAS is based on the SCSI command set. As a result,
the hardware will have to be changed but the system software
management infrastructure can be reused. SAS offers a superset
of features, so we should expect new extension features based
on existing SCSI software. The overall look and feel of SAS
will be similar to SCSI.

What
specifically will customers be able to re-use from their existing
SCSI infrastructure?
Existing system operating management software can
be reused. The higher number of Logical Unit Numbers (LUNs)
will be the main change. If the SAS drives are grouped in
the same number of drives per group as a SCSI bus, then there
will be even more compatibility.

Is
there a difference in system performance between a SCSI-based
system and a SAS-based system?
SCSI is a bus architecture, so the bandwidth of
320 Mbytes/s is shared between all the drives on that bus.
SCSI system performance is limited by the bus, and individual
drive performance is limited by the number of drives sharing
the bus. SAS is a point-to-point architecture, so the full
300 Mbytes/s is available to every drive. System performance
is only limited by the controller or the number of SAS channels
feeding the system.

Is
there expected to be difference in cost between SCSI-based
system and a SAS-based system?
The infrastructure costs should be similar. However,
in order to tap into the higher bandwidth available to each
drive, SAS systems will be able to offer higher performance
controllers for applications that require it. This provides
for a broad range of cost and feature support in SAS systems.

How
will IT managers and end users benefit from migrating to Serial
Attached SCSI?
The highly scalable nature of SAS allows the IT
manager to simply add more drives when required. The limit
is 128 drives per segment, as opposed to 14 drives with SCSI.
New segments are added with additional host controllers. Even
bandwidth can be scaled as a system grows.

What
do you feel will be the most challenging aspect of migrating
to SAS?
SAS is a new technology, and there is always a time
before the technology is widely trusted. In reality, the biggest
challenge will be to educate the customer base of the nearly
unbounded performance potential of SAS storage systems. SAS
performance can be tapped by higher performance controllers
and wide-pipe SAS channels. The current mindset is based on
the non-scalable SCSI architecture. SAS brings the opportunity
to customize the system performance for your requirements,
rather than having to align with the system bus speed.

Are
your end customers aware of Serial Attached SCSI and have
they started planning for upgrading to SAS?
I believe that many customers are aware of SAS,
especially the ones who would like higher performance. The
upgrade to SAS will be driven by the system feature enhancements,
and the scalability of SAS will help to protect the investment.

When
does Adaptec expect to see wide spread deployment of SAS systems
in the market?
The first systems will ship in 2004, but I don’t
expect the transition to be a quantum shift. SCSI systems
will be around for a long time yet, especially for markets
that do not need higher performance. 2005 will be the major
growth year for SAS, and it will become the default midrange
enterprise-class technology.

I think
the use of new small form factor 2.5” drives will be
a major step for SAS systems in 2005. This will allow greater
numbers of disk drives to be put into a smaller space. The
expandability of SAS systems, along with the scalability of
performance through the use of more powerful controllers,
will be a significant factor in the adoption of SAS.