Serial Attached SCSI: A Universal Interface
for Enterprise Storage Applications

Many factors determine a disk drive’s ability to meet a storage application’s requirements. Capacity, price, sustained data rates,
inputs and outputs per second (IOPS), and reliability under an application’s operating conditions are all criteria that systems
designers and IT mangers must consider when designing storage infrastructure. This white paper will examine these tradeoffs
and show how Serial Attached SCSI (SAS) is designed to enable a single storage system to meet a wide range of requirements.
High I/O Applications
The workloads for applications like databases, transaction processing, Web servers, and workgroup file servers typically consist
of transferring a large number of small blocks of data. These applications are considered I/O-intensive because their performance
is often gated by the I/O capabilities of the system’s disk drives. The number of IOPS each drive in the system can sustain
is critical to the performance of the storage systems serving these applications. Disk drives that rotate at 10K and 15K RPM are
designed to meet the needs of I/O-intensive applications.
The number of IOPS a drive can sustain is the inverse of the average access time. Or,
IOPS = 1/taverage access
The average access time for a disk drive can be calculated as follows:
taverage access = taverage seek time + taverage rotational latency + tcommand overhead
Let’s examine each of these components and their impact on a disk drive’s IOPS performance. The surface of a disk drive’s
media is segmented into a large number of circular tracks. Each track is further divided into a large number of sectors on
which data is stored. Seek time is defined as the time it takes the drive to move the head over to the track on which the data
is stored. Rotational latency is the time it takes the drive to rotate the media to the point where the data is located under the
head. The average rotational latency of a disk drive is equal to half of the time it takes for a drive’s media to make one complete
Command overhead is the time required for a drive to accept and process a command. Command overhead is typically
measured in microseconds, while seek time and rotational latency are measured in milliseconds. For random I/O applications,
the drive’s access time is usually approximated as just seek time plus rotational latency.
Serial Attached SCSI: A Universal Interface
for Enterprise Storage Applications
Kevin Gray
Figure 1
Serial Attached SCSI: A Universal Interface for Enterprise Storage Applications
Drives designed for high-I/O performance rotate at higher frequencies to reduce average rotational latency. To maintain
rotational stability and moderate power consumption, these drives use reduced-diameter media. The smaller media diameter
also shortens the distance the head must to move to position itself over the track on which the data is stored. Reducing this
distance lowers the seek time. High-performance drives use larger, more powerful magnets and smaller actuators to accelerate
the movement of the head during a seek operation. In disk drives designed for high I/O performance, these design differences
are combined to reduce average access time and increase I/O performance.
High-performance drives also use rotational position sensing to improve I/O performance. With simple tagged command
queuing, a disk drive will accept multiple read and write commands from the host and sort those commands in order to
minimize seek time. With rotational position sensing, the drive takes into account the angular position of the actuator as well
as the current track location. The drive then reorders read and write commands to minimize total access time. Rotational
position sensing requires additional processing power over algorithms that simply optimize seek time. However, this additional
processing power is used to improve the efficiency with which commands are sorted to improve overall I/O performance.
RAID controllers improve storage throughput by spreading I/O requests across multiple disk drives. As the number of disk
drives on a single controller grows, the I/O bottleneck often becomes the throughput capability of the RAID controller. Most
mid-range to high-end storage systems today are configured with redundant RAID controllers to improve system availability.
These systems typically offer dynamic load balancing as a means of achieving higher throughput.
Figure 2
Larger media increases
capacity but limits
rotational frequency
Smaller actuator improves
seek time
High IOPS drives use
larger magnets to
improve seek times
Figure 3—Rotational position sensing reorders I/O requests to reduce average access time.
Serial Attached SCSI: A Universal Interface for Enterprise Storage Applications
In storage systems with two or more active controllers that do not implement dynamic load balancing, groups of drives are
assigned to each controller. As workload conditions change, I/O requests can become skewed such that one controller is
overloaded with I/O requests while the other is operating at less than 100% capacity. Dynamic load balancing balances the
I/O requests across RAID controllers to utilize the full processing power of both controllers.
Dynamic load balancing requires that the installed drives have the capability to support I/O requests from more than one controller
at a time. This capability is referred to as supporting "multiple initiators." Disk drives that use the SCSI command set support
multiple initiators, while drives that use ATA commands do not. Parallel SCSI, fibre channel, and Serial Attached SCSI
drives all use the SCSI command set while Parallel ATA (PATA) and Serial ATA (SATA) drives do not.
Mid-Line and Streaming Media Applications
A disk drive’s duty cycle is the percentage of time the drive spends servicing I/O requests. This includes the time it takes the
actuator to move the head on track, rotate the media into position, and read or write the data. Under heavy I/O conditions, the
constant movement of the actuator along with current passing through the head affects the long-term reliability of the drive.
Disk drives that rotate at 10K and 15K RPM are built to operate reliably under high-duty-cycle workloads, 24 hours a day, seven
days a week.
Desktop ATA drives are designed to meet the requirements of consumer and desktop PC applications. These applications are
extremely cost sensitive. As a result, disk drive manufacturers have invested heavily in increasing the areal density of these
drives while lowering costs. The cost per gigabyte of desktop ATA drives has been reduced to the point where it is now less
than one third that of 10K RPM disk drives. Desktop PC and consumer applications typically see many fewer I/O requests and
are not required to operate 24 hours a day, seven days a week. Where 10K and 15K RPM drives are designed for 80% workloads
operating 24 hours a day, seven days a week, desktop ATA drives are designed for a workload with a 10% duty cycle
operating 12 hours a day, five days a week. The lower duty cycle and reduced operating hours enable desktop drive designers
to reduce the cost and increase the capacity of these drives to better serve their intended markets.
Maxtor has pioneered a new class of disk drives that targets an emerging class of "mid-line" storage applications. Mid-line applications
do not have the high I/O requirements of mainstream enterprise applications. Instead, they require large amounts of
capacity at a low cost per gigabyte while operating reliably in an enterprise environment. Maxtor leverages the high capacity and
lower cost per gigabyte of ATA disk drives through a specialized manufacturing process to create MaXLine™ disk drives, which
have very high capacities and are designed to operate 24 hours a day, seven days a week under a 20% duty cycle workload.
Figure 5—The Impact of Dynamic Load Balancing
50% - 50% 70% - 30% 90% - 10% 100% - 0%
With Dynamic Load Balancing
Load Skew
Without Dynamic Load Balancing
Figure 4
Video and streaming media applications require high sustained data rates to keep multiple streams of high-resolution video
and high-quality audio moving along at a crisp pace. Storage systems designed for these applications use RAID technology
to aggregate the data rate capabilities of a number of disk drives to meet the bandwidth requirements of these applications.
Although 10K and 15K RPM disk drives provide marginally higher sustained data rates than 7200-RPM ATA drives on a perdrive
basis, the cost of these drives is significantly higher. RAID arrays constructed of low-capacity, low-cost 7,200-RPM ATA
drives are now being used as a more cost-effective means of delivering the data rates required for video applications. The
cost per megabyte of sustained data rate for a low-capacity 7200-RPM ATA drive is almost half that of a 10K or 15K RPM
enterprise-class drive.
While 10K and 15K RPM drives provide the IOPs and reliability for mainstream enterprise applications, and high capacity
ATA drives provide the most raw capacity and best cost per gigabyte for mid-line applications, RAID arrays constructed
from low-capacity 7200 RPM ATA drives provide the most cost-effective means of delivering high sustained data rates for
video applications.
Serial Attached SCSI: a Universal Interface
Storage systems designed for SAS will support dual-port, 10K and 15K RPM drives for I/O-intensive applications, high-capacity
ATA drives for mid-line storage systems, and lower-capacity 7200 RPM ATA drives for video and streaming media applications.
The connector for a SAS drive is very similar to the connector on a SATA drive. SAS adds a second port to enable redundant
data paths for high availability configurations. SAS disk drives have been keyed to prevent them from being inadvertently
inserted into SATA storage systems. SAS system backplane connectors are compatible with both SAS and SATA disk drives.
At power up or after a system reset, SAS storage systems enter an initialization sequence during which drives are identified by
the system as either SAS or SATA drives and data transfer speeds are negotiated for each drive. If a drive is determined to be
SATA disk drive, the system’s controller communicates with the drive using ATA commands. If it is determined to be a SAS disk
drive, the controller communicates with it using the SCSI command set.
The ability to support both SATA and SAS disk drives enables Serial Attached SCSI storage systems to meet the widest range
of application requirements. This versatility simplifies storage system management by providing a universal building block that
can be deployed and redeployed as application requirements evolve over time. Database, email, Web servers, video servers,
and mid-line applications can all use a common enclosure to simplify the process of building and supporting a data center’s
storage infrastructure.
©2003 Maxtor Corporation. All rights reserved. Maxtor is a registered trademark, and What drives you is a trademark, of Maxtor Corporation.
Maxtor Corporation, 500 McCarthy Boulevard, Milpitas, CA 95035. WP--SASUniversalInterface-10/03-CL
To learn more about Maxtor Corporation, visit
Serial Attached SCSI: A Universal Interface for Enterprise Storage Applications
40GB 7200RPM
Desktop ATA
36GB 10K RPM
18GB 15 RPM
$2.64 $2.82
Figure 6— $/MB Sustained Data Rate


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