Author:
Paul Griffith
Adaptec, Inc.
Today’s
system builders must develop, test, qualify, inventory and
sell separate subsystem backplanes to meet their customers’
needs for storage of reference and transactional data. Distributors
and IT managers must also deal with the inefficiencies of
separate systems. Serial Attached SCSI (SAS) will enable computer
makers to build a single system that supports disk drives
for both data types, reducing development costs. For customers,
standardizing on one server and storage infrastructure will
increase storage flexibility and reduce total ownership costs.
Developing
a Single Storage Backplane—SAS Flexibility
A key advantage of SAS is that its backplane design and protocol
interface allow the use of both SAS and SATA disk drives in
the same system, giving system builders the flexibility to
tailor workstation or server storage to the data type. Though
each drive type is typically used in different applications,
most enterprise users have needs for both.
To ensure
storage flexibility, the SAS backplane and cable connectors
are supersets of the SATA versions, with a second port for
failover redundancy. SAS links will transport both SCSI and
ATA commands. ComSAS unites SATA’s OOB (out-of-band) signals,
ComInit and ComWake, to enable communication and negotiation
between initiator controllers and target peripherals to determine
whether each of those peripherals is SATA or SAS. The same
differential wires transmit and receive OOB signals and normal
drive command and data traffic. OOB signals have easily distinguishable
lower frequency characteristics; out-of-tolerance PHYs can
encode and decode these signals, which are present only when
the SATA or SAS link is inactive.
SAS offers
additional flexibility in its design as both a multi-target
and multi-initiator controller and supports a wide output
voltage range, enabling its use as a lower cost alternative
fabric to Fibre Channel in longer backplane and near-enclosure
cabling configurations.
Benefits
of Compatibility
SAS and SATA compatibility offers a number of benefits to
system builder 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 will be as simple as replacing
the disk drives. With parallel technologies, upgrading from
ATA to SCSI means replacing backplanes, connectors, cables,
RAID controllers and drives. SAS support for both SAS and
SATA drives will reduce system builder costs substantially
by simplifying procurement, validation and inventory management.
VARs
and System Integrators will be able to easily configure
custom systems by simply installing the appropriate disk drive,
as 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 without having to buy a new system
will greatly simplify the purchasing decision, future-proof
system investment, and reduce the total cost of ownership.
