Serial Attached SCSI Architecture Deployment Models

Author: David Allen, Director of Strategic Marketing, Storage Products Division
Vitesse Semiconductor

Architecture Deployment

Attached SCSI (SAS) expander architectures provide flexible,
cost-effective connections for internal and external storage
subsystems in the mid-range and enterprise server markets.
In this article we address SAS expanded architectures applicable
to both server and storage enclosure applications.

Types and Features

SAS Edge
Expanders and Fan-out Expanders provide switching for connecting
multiple targets to a host connection. Edge expanders support
up to 128 SAS PHYs. Similarly, “Edge Expander Device
Sets” can be constructed from multiple edge expanders.
Direct connections, subtractive connections, and limited table
routing enable communication between individual expanders
(see Figure 1). Because they employ limited table routing,
edge expanders are targeted at low-cost designs with limited
storage scalability requirements.




fan-out expanders are more scalable, providing multiple tiers
of expansion capability. They can also determine the devices
to which they are physically connected and understand the
topology of all downstream devices. Because fan-out expanders
are often used in large, hierarchal external storage subsystems,
they employ large internal routing tables for addressing multiple
expanders and targets beyond the expander’s physical
connections. The number of expanders and targets a fan-out
expander can support, is limited only by its table routing
capability, and ultimately the SAS-specified limit of 16,384


To enable
remote system monitoring in SAS architectures, both edge and
fan-out expanders usually include either an integrated Enclosure
Management (EM) capability or a method to communicate with
an external EM device. EM devices enable system diagnostics
and report the status of the expander itself and related external
system components, including fan-speed and pulse width modulation controls. Often
multiple general purpose I/Os are incorporated which provide
LED and system control.

Architectural Models

Expanded Architectures In Direct Attach Storage (DAS) Models
SAS HDDs and targets are primarily attached to the host
by two methods. The simplest model allows for “direct”
connection to the host via a host controller without expansion
(see Fig. 2).




In this
model, scalability and physical connections are limited to
the number of PHY ports on the controller. Similarly, bandwidth
is not aggregated in the direct-connect model, so the improved
performance of the SAS controller goes unused.

The second
approach employs an “expanded” architecture that
allows for a more flexible and scalable design that can take
advantage of the bandwidth available through the SAS host
controller. The expanded model provides for significant flexibility
in both server storage and storage subsystem architectures
(see Fig. 3).




System Failover Techniques

SAS expander designs are also capable of providing fault tolerant
mechanisms within a single host or between multiple hosts.
In the case of Figure 4 below, failover is achieved via the
connection scheme and the use of dual-port drives. This approach
provides maximum data availability and protection for the




Expanded Architectures in Scalable Storage Subsystems

Large storage subsystems are primarily designed through the
use of high-port-count expanders. These devices are capable
of addressing a large number of dual-port SAS drives. Systems
employing these architectures provide robust fault tolerance
alternatives to storage arrays, depending on the specifics
of the implementation. In most cases, external connections
are provided to allow for storage subsystem scalability. As
the storage system grows, the root expanders provide the addressing
capability needed to connect all devices downstream. Figure
5 depicts a simple subsystem with failover capability.




Economical Deployment

architectures within a SAS deployment model allow for storage
scalability, and can address two distinct economic models
with the same design. An enterprise-level system is enabled
by the selection of SAS HDDs, with the end user receiving
all the expected performance and reliability provided by similar
parallel SCSI systems today. A second economic model is achieved
by utilizing the same SAS architecture but by deploying Serial
ATA (SATA) HDDs. SATA drives provide an economical storage
alternative with some reduced functionality and performance
capabilities. A third economic model is possible where both
interfaces can be simultaneously deployed in the same SAS
infrastructure, providing even greater storage flexibility
to the user.


As with
any technology, the future will ultimately be determined by
the needs of the consumer. Speed increases and functionality
are sure to be on the short list as SAS architectures mature.

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