introduction to Serial
Attached SCSI (published here in January 2005) gives you an idea of the
performance, compatibilities, applications and roadmap for this new directly
attached disk connection standard. With throughput capability faster than 2Gbps
Fibre-channel systems and
faster than ultra320 SCSI
- the new SAS products not only provide an upward migration path for parallel
SCSI applications but also open the door to a new class of high performance
high reliability enterprise systems. |
|The Benefits of SAS (Serial
Attached SCSI) for External Subsystems|
Serial Attached SCSI
(SAS), the follow-on to parallel SCSI, is designed for high performance
enterprise requirements and offers both the benefits of backward compatibility
with SCSI and interoperability with Serial ATA (SATA), bringing
enterprises a flexibility and cost savings previously not possible for
traditional storage environments. SAS provides significant benefits to external
storage subsystems and offers users "one-stop-shopping" to satisfy
their requirements for the following three main data types.
- High MB/s and large data intensive files
- Large block, random read/writes
- Video streaming
- Maximum IOPs for OLTP, calculation intensive files
- Small block, random read/writes
- Reservation systems
- Billing systems
- Fixed content, archival data for secondary / nearline
- Large block, sequential writes / reads
- Medical records
|An external subsystem with a
SAS midplane will support both high performance, reliable SAS drives ideal for
transaction data, and cost-effective, high capacity SATA drives ideal for
reference and throughput data, thus providing customers with one platform
instead of two to support both drive types and satisfy all three application
segments (illustrated in figure 1). SAS based external storage arrays provide
benefits in four areas: performance, availability, flexibility, scalability and
cost of ownership. |
Because of its trusted reliability and stable feature set, the SCSI
protocol has been accepted by the market for a remarkable twenty years.
Introduced three years ago, Serial Attached SCSI (SAS) leverages this continuing
evolution of SCSI with new levels of scalability, flexibility, and
cost-effectiveness for connectivity, data transport, and data storage.
The first SAS prototypes were announced in 2003 and were a major step
to achieving mass market availability. Those prototypes allowed development of
the first generation of technologies and products that will bring the benefits
of SAS into the enterprise. These products have now been developed and tested,
and a wide variety of integrated solutions have been demonstrated.
Interoperability testing was a key component of SAS, because it
increases the architecture's flexibility by supporting both SAS and Serial ATA
(SATA) disk drives and components. Interoperability allows one vendor's SAS
products to be compatible with another's, and it also ensures products developed
today will work with all existing and next-generation SATA products.
Interoperability testing was conducted throughout 2004, and the
University of New Hampshire InterOperability Laboratory, successfully
demonstrated all the required levels of interoperability in a variety of SAS
products and configurations.
The same companies that pioneered SCSI are investing in its evolution
to maintain its high reliability standards while meeting the market's evolving
technology and functionality requirements. Industry support comes from all of
the major disk drive vendors, host adapter suppliers, chipset manufacturers,
large computer makers, and many other suppliers.
A wide variety of recent product and solution announcements have been
made, with some product currently shipping. Others are expected to ship in early
2005 to make the long promise of SAS a reality. SAS subsystems are expected to
ship in the first quarter of calendar 2005, and will coincide with the
availability of SAS disk drives and SAS host bus adapters. Volume shipments
will commence in the second quarter.
SAS has received this kind of market attention due to several
performance benefits. For example, a SAS connection can support four SAS wide
lanes or 4 x 300 MB/s per connection.
A SAS connection on a
JBOD system can support a
theoretical maximum of 1,200 MB/s (see figure 2). SAS is also used as a high
performance, yet cost-effective expansion port to daisy chain to another SAS
|In addition, each 3Gb high
performance SAS drive (10K rpm or 15K rpm) is connected to a 3Gb drive
connection, providing more then enough bandwidth for the highest performing
drives (see figure 2). Assuming 16 SAS drives in a subsystem and each drive
capable of 75 MB/s, that would equal 1,200 MB/s performance from 16 drives which
far surpasses both 2Gb Fibre Channel loops and U320 Parallel SCSI buses. The
roadmap for SAS is robust and provides clear investment protection by starting
at 3Gb, doubling to 6Gb in 2007 and then doubling again to 12Gb.|
Another benefit of SAS based storage subsystems is the native
dual port capability of each SAS drive, providing a redundant path to each drive
in the event of a controller failover. In addition, since each drive is on a
separate point-to-point connection, losing a drive connection only affects a
single drive compared to parallel SCSI in which losing a bus affects all the
drives on the bus.
SAS drives like Fibre Channel are designed for
the rigors of enterprise use and heavy loads, have MTBF ratings in excess of 1
million hours and warranties up to 5 years.
SAS drives are engineered
for rugged enterprise duty, and every component (drive motor, spindle, actuator,
firmware, etc.) is specifically designed and manufactured for that rigorous use.
SAS drives also safeguard data integrity via their comprehensive
verification/error correction capabilities.
SAS (drive) based subsystems also support active-active controllers
that failover, redundant host connections that could be SAS, Fibre Channel or
iSCSI, redundant hot swappable power, cooling, and enclosure services and the
same RAID and software capabilities found in Fibre Channel and SCSI based
Probably the best known benefit of SAS for external storage is its
configuration flexibility. The ability to take a SATA drive and plug it into a
SAS midplane without any modifications or changes, and for the controller in the
array and software to seamlessly support a system with a mix of SATA and SAS.
As illustrated in figure 3, 6 SAS drives could be partitioned in one storage
pool or LUN and assigned to a server with a transaction intensive application
such as a reservation system, and the other 6 drives be SATA and partitioned in
a storage pool or LUN and assigned to a different server running a reference
application such as medical imaging. Alternatively, one array could support all
SAS drives and a second array connected or daisy chained to the first supporting
all SATA drives. This flexibility is not possible with Fibre Channel and
Parallel SCSI based subsystems providing customers with the best of both worlds.
|Seamless Scalability &
Improved Cost of Ownership
Unlike SCSI which has limitations in its architecture when trying to
scale multiple drive enclosures, SAS provides the capability to scale greater
than 100 drives or 36 TB with 300GB SAS drives or 48 TB with 400GB SATA drives
providing very large pools of storage and a cost-effective alternative to Fibre
Channel based SANs.
SAS drives are expected to be priced on par with SCSI drives, and SAS
implemented as a host interface and expansion interface on a RAID controller or
JBOD I/O module will be less costly then Fibre Channel. In addition, supporting
SAS and SATA drive types in one system reduces cost of ownership, simplifies
training and product portfolios and provides investment protection.
Serial Attached SCSI will offer new levels of performance,
availability and customer choice by supporting both enterprise-class Serial
Attached SCSI drives and Serial ATA drives for cost-sensitive applications. OEMs
and IT managers will have the flexibility to configure storage subsystems with
either drive technology, or both, enabling high-performance and low-cost storage
in the same subsystem thus maximizing customers' return on investment and
providing flexibility for future growth.
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|Users now realize that in
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they get away from the diverse feature sets of any single supplier towards a
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market consolidation? - everything will change|
|sugaring MLC for the
|When flash SSDs started to be used as
enterprise server accelerators in 2004 - competing
RAM SSD makers said
flash wasn't reliable
RAM SSDs had been used for server speedups
- and in 2004 they owned the enterprise market. (Before 2004 - flash SSDs
weren't fast enough and had mostly been used as rugged storage in the
markets - and in space
constrained civilian products such as smartphones.)
By 2007 it was
clear that the endurance
of SLC flash was more than good enough to survive in high
caches. And in the ensuing years the debate about enterprise flash SSDs shifted
to MLC - because when systems integrators put early cheap consumer grade SSDs
into arrays - guess what happened? They burned out within a few months - exactly
Since 2009 new
technologies and the combined market experience of enterprise MLC pioneers
like Fusion-io and
demonstrated that with the right management - MLC can survive in most (but
still not all) fast SSDs.
Now as we head into 1X nanometer flash
generations new technical challenges are arising and MLC SSD makers disagree
about which is the best way to implement enterprise MLC SSDs.
type of so called "enterprise MLC" is best? Can you believe the
contradictory marketing claims? Can you even understand the arguments? (Probably
And that's why marketing is going to play a bigger part in the
next round of enterprise SSD wars as SSD companies wave their wands and reveal
more about the magic inside their SSD engines to audiences who don't really
understand half of what they're being told.