Serial Attached SCSI (SAS) and SATA: Compatible, Not Interchangeable

Franco Castaldini & Kent Bransford
Seagate Technology

The advent
of Serial Attached SCSI coincides with a fundamental change
in enterprise data management. Gone are the days when there
were only two types of data storage: online and offline. A
new concept, near-line or secondary storage, quickly gained
acceptance because it recognizes that data has a lifecycle,
during which its need for accessibility and security will
vary. Rather than a simple online/offline dichotomy, enterprise
storage now comprises a continuum along which data can easily
migrate as its value and relevance change.

This enterprise
storage continuum gives IT managers unprecedented freedom
to select the most appropriate, targeted solution for a given
task, helping to ensure both higher performance and greater
cost effectiveness.




Solution Application Notes
high availability, random reads
for mission-critical data in large enterprises [e.g.,
Storage Area Network (SAN)], supports up to 16 million
addresses and maximum cabling distance of ten kilometers,
relatively costly
Attached SCSI (SAS)
high availability,random reads
for mission-critical data in mid-size enterprises, high
performance coupled with exceptional scalability on local
level, more affordable than FC, compatible with SATA
low availability,sequential reads
capacity at low cost, not suitable for mission-critical
data, compatible with SAS
Offline High
capacity, ability to move data offsite for archival and


and SATA: Ideal Complements

Serial Attached SCSI’s compatibility with Serial ATA
(SATA) brings an added dimension of flexibility to the enterprise
storage continuum. Because SAS and SATA drives can share a
common backplane and be housed in a single enclosure, a single
SAS-based subsystem is capable of handling a broad range of
enterprise storage duties, from high-availability online storage
to data migration, disk-to-disk backup and near-line storage.
Consolidating drives into fewer, more rationalized subsystems
can save a substantial amount of both money and space, as
well as reducing IT storage maintenance costs.

growing firms can purchase SAS cables, backplanes and enclosures
for initial use with SATA drives only, secure in the knowledge
that such equipment will not become obsolete when their storage
needs expand and enterprise-class drives are needed. SAS drives
can simply be plugged into their existing storage subsystems.

Interface, Different Solutions

To most efficiently deploy SAS and SATA drives, it’s
important to appreciate the fundamental differences between
these two serial-based storage solutions. SAS was designed
and engineered for high availability, enterprise-class data
storage where random-read performance, reliability and data
integrity are absolutely paramount. SATA is a desktop-class
solution that offers high capacity at low cost, and impressive
throughput on sequential reads (e.g. backup and restore operations).
The key is to remember that SAS and SATA complement each other,
and that their compatibility in no way suggests that they
are interchangeable.

easy to identify instances in which moving data from SAS to
SATA would be beneficial. For example, using SAS drives for
bulk storage of non-critical, low-availability data is a wasteful
allocation of resources, and SAS backplanes will enable IT
managers to seamlessly substitute inexpensive SATA drives
for such near-line storage applications. That constitutes
a sensible, cost-effective use of SATA drives in an enterprise

under continued pressure to trim IT budgets and flushed with
enthusiasm for SATA’s apparent price/performance advantage,
some IT managers have begun deploying SATA drives in enterprise
roles for which they are wholly unsuited. Any savings realized
by choosing SATA over SAS can quickly be erased, many times
over, by the cost of lost data, downtime, reduced productivity,
etc. should SATA drives fail in the severe-duty environments
for which SAS drives are expressly designed.

Make no
mistake, though their serial interfaces make them appear quite
similar, SAS and SATA drives are vastly different. SAS drives
are engineered for rugged enterprise duty and every component
(drive motor, spindle, actuator, magnetic recording heads,
control and servo processors, firmware, etc.) is specifically
designed and manufactured for that rigorous use. SATA drives
are impressive examples of clever engineering to achieve great
capacity at low cost, but they are in no way capable of meeting
the performance and reliability standards demanded in high-availability
enterprise storage applications:


Device Duty
(Typical )
Time (Typical )
(15K rpm)
7 days/week
(7200 rpm)
5 days/week


Right Tool for the Job

Enterprise storage has entered a new era in which targeted
solutions are now available to address a broad variety of
needs, from the most rigorous high-availability applications
to cost-sensitive bulk data storage. Selecting the “best”
storage device is simply a matter of ascertaining needs, evaluating
the capabilities and limitations of each potential solution,
and then choosing the most appropriate tool for the job. Serial
Attached SCSI and Serial SATA both have their place in the
enterprise storage continuum, and SAS’s compatibility
with SATA further enhances the flexibility and appeal of these
two complementary technologies.

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