THE R/EVOLUTION DIFFERENCE
white paper
SAS vs. FC
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
ª
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
ª
•
•
•
•
•
•
•
•
•
SAS & FC
COMPARED
white paper
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
ª
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
ª
•
•
•
•
•
•
•
•
•
TABLE OF CONTENTS
Introduction 3
SAS (Serial Attached SCSI ) - the New SCSI 3
Market Timing 3
SAS and FC Similarities 4
SAS Benefits 5
Combining the Best of Both Worlds 5
End-to-End FC Interface Removed 6
Utmost Flexibility 6
Seamless Scalability & Improved Cost of Ownership 6
The Nature of SAS and FC Drives 6
Will SAS Upstage Fibre Channel? 7
SAS Roadmap 7
Conclusion 8
THE R/EVOLUTION DIFFERENCE
SAS vs. FC
SAS & FC
COMPARED
Introduction
In today's economy, cost-effective, per-
formance-oriented, scalable and reliable
storage solutions are fundamental to suc-
cess. Today's IT managers as well as
storage vendors need to keep their com-
petitive edge by continuing to reduce the
cost of their operations and solutions. No
matter what the company size, the
process of scrutinizing access time to
data is occurring. As the value to the
access of data increases the utilization of
storage resources, it also exposes the
issue that not all data is the same. Often,
this is a function of the age of data. IT
managers are looking for more cost-effec-
tive, high-performance storage, as well as
to maximize the efficiency of their IT
spending.
This paper explores SAS -- a relatively new
entry into the disk drive market -- as a
very viable replacement for Fibre Channel-
based (FC) disk drives. Although it is not a
complete replacement for FC, SAS will
eventually come to command the
midrange disk market for external stor-
age.
SAS (Serial Attached SCSI ) - the
New SCSI
Foreseeing the end of the traditional SCSI
drive interface, a group of storage indus-
try leaders, including representatives
from system, controller, and hard drive
companies, met to define a new interface
that would become known as Serial
Attached SCSI, or SAS. The conversion of
Parallel SCSI to SAS started in 2002. In
2003, the ANSI standard was passed
allowing the industry to initiate a new
beginning. Starting roughly, where the
Parallel SCSI ended, at 320Mbps per-
formance, SAS is an improvement by
being a point-to-point, full duplex, dual-port
interface. Although cable lengths are
restricted to 10 meters with no optical
capability, SAS permits simultaneous input
and output transfers to be full duplex
active at each communicating port.
SAS will ultimately replace traditional
Parallel SCSI drives, which have been
around for the better part of two decades
and have reached their performance
limit. Parallel interfaces have become
technologically more challenging as their
respective clock frequencies have
increased to keep pace with the band-
width requirements of their attached stor-
age devices. Introduced in 1986, the
SCSI interface offered an 8-bit data bus.
Improvements expanded it to a 16-bit
data bus, leaping the performance from
10Mb per second to 20Mb per second
without increasing the clock speed. In
2001, the maximum clock speed for
SCSI attained 320 MHz (maximum burst
rate of 320MB per second) after over-
coming significant obstacles such as sig-
nal distortion ("cross-talk") and skewing
errors. In addition, unwieldy cables, the
prevention of skewing errors and signal
distortion further contributed to the
demise of the parallel interface - further
development ceased. To gain more head
room for performance and longevity, SAS
is now SCSI's future
Market Timing
Because of its trusted reliability and sta-
ble feature set, the SCSI protocol has
been accepted by the market for a
remarkable 20 years. The SCSI interface
has long been the backbone of perform-
ance-based systems but has all but run
out of steam due to its parallel architec-
ture. Introduced three years ago, Serial
Attached SCSI (SAS) leverages this contin-
uing 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 achiev-
ing mass market availability. Those proto-
types allowed development of the first
3
SAS & Fibre Channel - Compared
0.0
5,000.0
10,000.0
15,000.0
20,000.0
25,000.0
30,000.0
35,000.0
40,000.0
45,000.0
2005 2006 2007 2008 2009
Gartner HDD Interface Share Forecast
SATA (Multiuser) Total Unit s
FC-AL Total Units
SAS Total Units
Parallel SCSI Total Units
Figure 1. Source Gartner 2005
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 compo-
nent of SAS, because it increases the
architecture's flexibility by supporting both
SAS and Serial ATA (SATA) disk drives
and components. Conducted throughout
2004, the University of New Hampshire
InterOperability Laboratory successfully
demonstrated all the required levels of
interoperability in a variety of SAS prod-
ucts and configurations. 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.
The same companies that pioneered SCSI
are investing in the evolution of SAS 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.
In 2005, SAS disk drives shipped in very
modest quantities, yet are anticipated to
capture a majority of the market within
the next two years (note Figure 1).
Experts are predicting SAS drives will be
the hottest storage media to come along
since Serial ATA (SATA), its lower-end
counterpart, while offering a lower-cost
alternative to Fibre Channel drives, its
higher-end counterpart. SAS is likely to
become a standard by the end of 2006,
much like SATA replaced ATA drives on all
but the lowest-end PCs.
SAS and FC Similarities
Both FC and SAS offer the maturity, rich-
ness, depth and scope of the SCSI com-
mand set. They are differentiated by their
drive-to-drive connectivity, their inter-box
connectivity and their addressability. FC's
combination of shared media access,
data rate, optical support and fabric com-
patibility has made it the interface of
choice for SANs and high-speed switching
environments.
The newest FC drives available today sup-
port 4 Gbps
1
a data transfer rate of up
to 400 MBps half-duplex and up to 800
MBps full-duplex, per port. On the heels
of 4 Gb FC, SAS has received a great
deal of market attention due to several
performance benefits. Supporting data
transfer rates of up to 3 Gbps and full
duplex, point-to-point connections (so that
each drive has a dedicated connection to
the host or RAID controller), a SAS con-
nection can support four wide lanes or 4
x 300 MB/s per connection. SAS will
compete with FC in all but the most highly
data intensive transactional applications.
For example, 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 subsystem.
SAS drives, like Fibre Channel drives, 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. Both SAS and FC drives
are engineered for rugged enterprise
duty, and every component (drive motor,
spindle, actuator, firmware, etc.) is specif-
ically designed and manufactured for that
rigorous use. SAS drives also safeguard
data integrity via their comprehensive veri-
fication/error correction capabilities.
1. The 4Gb FC interface is backward compatible
with 2 Gb FC.
Figure 2. R/Evolution SAS Architecture
4
SAS & Fibre Channel - Compared
SAS drive-based subsystems also support
the following:
"Active-active controllers for failover
"Redundant host connections that could
be SAS, Fibre Channel or iSCSI
"Fibre Channel or iSCSI
"Redundant hot swappable power, cooling,
controllers and disks
"Enclosure services and the same RAID
and software capabilities found in Fibre
Channel and SCSI-based arrays.
Given the superior price/performance
ratio and reliability equivalence to FC
drives, unit sales for SAS drives will grow
significantly in the next few years. Overall
unit shipments for SAS will grow modest-
ly, while FC drives units will remain rela-
tively flat with SATA drives showing contin-
ued growth as well.
SAS Benefits
Relative to parallel SCSI, Serial Attached
SCSI drives (SAS) will have much more
throughput and higher levels of perform-
ance. Most notably, SAS drives can send
and receive data simultaneously or serial-
ly. SAS drives support speeds of up to
15,000 RPM; double that of SATA
drives.
From a cost perspective, SAS drives will
be priced similarly to Parallel SCSI drives,
and be less expensive than Fibre Channel
drives and related infrastructure. Fibre
Channel will not phase out. However, SAS
will become the drive of choice for most
performance related applications. In addi-
tion, each 3 Gb high performance SAS
drive (10K rpm or 15K rpm) is connect-
ed to a 3Gb drive connection, providing
more than enough bandwidth for the high-
est performing drives. SAS features a
point-to-point architecture, in which all
storage devices connect directly to an
expanding switching matrix rather than
sharing a common bus as traditional SCSI
devices do, essentially increasing data
throughput and improving the ability to
locate and fix disk failures. SAS inherits
its command set from parallel SCSI, its
frame formats from Fibre Channel, and
its physical characteristics from Serial
ATA.
Combining the Best of Both
Worlds
Until recently, attaching multiple Fibre
Channel hard drives in a RAID system via
FC met nearly all performance require-
ments for many IT environments. Then
the concept of tiered storage became
more tangible when SATA hard drives
were introduced, offering much higher
capacity and lower cost though lower per-
formance. SATA drives began to replace
SCSI drives as a result. The decision
WAS simple: choose FC hard drives for
applications that require any type of per-
formance; then choose SATA hard drives
when low cost storage and/or high
capacity is required. The addition of SAS
has increased the options IT managers
have to solve their disk storage chal-
lenges providing an alternative solution for
high performance applications with an
attractive price/performance. SAS will
garner most of the market for external
storage. Fibre Channel disk drives, as
Gartner and IDC have forecasted will be
found installed in applications where per-
formance requirements exceed the price-
to-performance attributes SAS offers.
SATA will continue to be the storage
choice for economic driven applications.
Figure 3 SAS Expander
5
SAS & Fibre Channel - Compared
FC
SAS
SATA
Highly Available, Very Frequent Transactions
(required by very few applications)
Superior Price/Performance Ratio
General to Performa nce Oriented
Storage Demands
(majority of applications)
Low Frequency/Availability
(tape alternative for backup)
FC
SAS
SATA
End-to-End FC Interface Removed
When integrated into a RAID system, the
SAS disk drive will seamlessly integrate
into FC networked environments.
Historically, a homogeneous interface
from the host to the device was a
requirement for many IT departments.
For instance, when the communication
initiated with Fibre Channel at the host,
the storage device native interface was
normally Fibre Channel due to real, as
well as unfounded, perceptions of interop-
erability. As discussed earlier, the SAS
drive has overcome interoperability barri-
ers and the prevalence of RAID con-
trollers attached via Fibre Channel will
continue. However, within the RAID enclo-
sure, a paradigm shift will soon occur
with SAS drives as primary storage and
FC-to-SAS conversion occurring on the
RAID controller. As mentioned earlier,
leveraging SAS expanders within the RAID
system is the back-end method for clus-
tering SAS disk drives. RAID systems with
SAS drives will be completely transparent
to hosts when attached via Fibre Channel.
Utmost Flexibility
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. A portion of SAS drives
can be partitioned into one storage pool
or LUN and assigned to a server with a
transaction-intensive application such as a
reservation system, and then another
portion of SATA can be partitioned into a
storage pool or LUN and assigned to a
different server running a reference appli-
cation such as medical imaging.
Alternatively, one array could support all
SAS drives and a second array connected
(or daisy chained) to the first one could
support all SATA drives. This flexibility is
not possible with Fibre Channel and
Parallel SCSI-based subsystems - SAS
provides customers with the best of both
worlds.
Figure 4. reflects the breadth that the
SAS technology offers: the SAS backplane
and infrastructure allows for SATA proto-
cols to travel over the same wires. Six
drives can be SAS drives and six drives
can be SATA within one RAID enclosure.
Amplifying this flexibility, the RAID con-
troller could have the option of attaching
to one of three networks via a 4Gb FC,
iSCSI or 3Gb SAS HIM (Host Interface
Module).
Seamless Scalability & Improved
Cost of Ownership
Unlike SCSI, which has limitations in its
architecture when trying to scale multiple
drive enclosures, SAS provides the capa-
bility to scale greater than 100 drives or
36 TB with 300GB SAS drives or 56 TB
with 500GB SATA drives, providing very
large pools of storage and a cost-effective
alternative to Fibre Channel-based SANs.
Essentially the components that allow the
device to communicate over the FC net-
work are more costly. 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.
The Nature of SAS and FC Drives
Designed for high performance enterprise
requirements and offers both the benefits
of backward software compatibility with
SCSI and interoperability with Serial ATA
(SATA), Serial Attached SCSI (SAS) deliv-
ers the flexibility and cost savings previ-
ously not possible with traditional storage
environments. SAS provides significant
benefits to external storage subsystems
and offers users "one-stop-shopping" to
satisfy their requirements for three main
data types: throughput, transaction and
reference, highlighted in Table 1.
Figure 4 SAS/SATA Flexibility
6
SAS & Fibre Channel - Compared
4Gb FC
iSCSI
or
SAS Drives
•
Higher Performing
•
Reliable
•
Mainstream
•
Dual ported
•
10K and 15K rpm
3Gb SAS
or
or
SATA Drives
•
High Capacity
•
Cost Effective
•
Nearline Apps
•
Single Ported
•
Dual Ported (w/ Mux
•
7,200 rpm
)
))
7
An external subsystem with a SAS mid-
plane will support both high-performance,
reliable SAS drives, which are ideal for
transaction data; and cost-effective, high-
capacity SATA drives, which are ideal for
reference and throughput data. This pro-
vides customers with one platform --
instead of two -- to support both drive
types, thus satisfying all three application
segments illustrated above. SAS-based
external storage arrays provide benefits in
these five areas: performance, availability,
flexibility, scalability and total cost of own-
ership, or TCO.
Will SAS Upstage Fibre Channel?
SAS will be the catalyst to the scrutiny
that will occur in IT environments debat-
ing the value of access to data. Given the
superior price-to-performance ratio, over-
taking the performance oriented storage
position Fibre Channel has dominated will
be certain for SAS. Indeed, there will con-
tinue to be a minority of applications such
as online transaction processing and data
analysis, Fibre Channel will have a positive
impact - airline computer ticketing sys-
tems ten minutes before takeoff. Hence,
the number of unique applications Fibre
Channel drives will diminish as prudent IT
managers replace SAS based storage for
most performance-oriented environments.
SAS Roadmap
The SAS roadmap outlines robust growth
and investment protection attributes by
starting at 3 Gbps, doubling to 6Gbps in
2007 and then doubling again to
12Gbps.
SAS Hard Drive with FC Interface Applications
SAS & Fibre Channel - Compared
• Extremely high MB/s and
large data intensive files
• High volume - large block,
random read/writes
• High res. video streaming
• Collaborative CAD ses-
sions
• Maximum IOPS for
OLTP, calculation intensive files
• High volume - small
block, random
read/writes
• High volume reserva-
tion systems
• Billing systems
• Fixed content,
archival data for secondary/ nearline
storage
• Large block, sequential writes/reads
• High res. imaging
• Medical records
Throughput Data Transaction Data Reference Data
FC Hard Drive with FC Interface Applications
2006
2007
2008
2009
2010
2011
2012
2013
Ultra 320 SCSI
SAS 3Gb/s SCSI
SAS 6Gb/s SCSI
SAS 12Gb/s SCSI
Source:
www.scsita.org
2006
2007
2008
2009
2010
2011
2012
2013
Ultra 320 SCSI
SAS 3Gb/s SCSI
SAS 6Gb/s SCSI
SAS 12Gb/s SCSI
Source:
www.scsita.org
Figure 5 SAS Roadmap
Table 1. FC-SAS and FC-FC Applications
• High MB/s and large
data intensive files
• Large block, random
read/writes
• Low/med res. video
streaming
• CAD
• Maximum IOPS for
OLTP, calculation intensive files
• Small block, random
read/writes
• Reservation systems
• Billing systems
• Fixed content,
archival data for secondary/ nearline
storage
• Large block, sequential writes/reads
• Imaging
• Medical records
Throughput Data Transaction Data Reference Data
Conclusion
Serial Attached SCSI, SAS, will offer new
levels of performance, availability and cus-
tomer choice by supporting both enter-
prise-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' total return
on investment and providing flexibility for
future growth. Furthermore:
• SAS will replace parallel.
• SAS will replace direct-attached FC
• SAS will replace FC in external RAID
systems (except for high-end SANs)
• Many applications will support SAS and
SATA drives within the same system
As the sophistication of evaluating the
value of data improves, IT managers will
continue to scrutinize the requirement for
data to be always available at extremely
high data rates. While the FC network
continues to be a mainstay, expensive
ultra high performance Fibre Channel disk
drives will apply to only niche applications.
Parallel SCSI disk drives will quickly
become extinct in place of very cost-effec-
tive disk storage. For the very low cost,
high capacity disk drive choice, SATA disk
drives will occupy that market space. A
great balance between capacity and per-
formance SAS is the natural successor to
parallel SCSI, just as SATA is to parallel
ATA.
Further, SAS will all but replace Fibre
Channel disk drives as IT managers real-
ize the superior price to performance
SAS disk drives offer. While offering alter-
natives to more costly high-end systems,
SAS will provide a new tier of storage in
the midrange.
SAS will be the foundation of high-end
enterprise storage over the next several
years. It's the key ingredient to enabling
new approaches to tiered storage.
8
SAS & Fibre Channel - Compared
© 2006 Copyright Dot Hill Systems Corporation. All rights reserved. Dot Hill Systems Corp., Dot Hill, the Dot Hill logo and
SANnet are trademarks of Dot Hill Systems Corp. Product names mentioned herein are trademarks and/or registered trademarks of their respective owners. Specifications and features may change without notice. 09-15-05
Corporate Headquarters:
2200 Faraday Avenue
Suite 100
Carlsbad, CA 92008, USA
800-872-2783
760-931-5500
Fax: 760-931-5527
Europe:
Marssteden 94
7547 TD Enschede
THE NETHERLANDS
31-53-428-4980
Loading...