1. Direct Attached Storage and Introduction to SCSI
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Section 2 : Storage Networking Technologies and Virtualization
Direct Attached Storage and Introduction to SCSI
Chapter 5
Module 2.1 – Direct Attached Storage and Introduction to SCSI

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Chapter Objective
Upon completion of this chapter, you will be able to:
Discuss the benefits and challenges of DAS
Discuss DAS management options
Discuss evolution of SCSI
Describe SCSI – 3 architecture
Discuss SCSI addressing and communication model

3. Lesson: Direct Attached Storage
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Lesson: Direct Attached Storage
Upon completion of this lesson, you will be able to:
Discuss the benefits of DAS
Describe the elements of DAS
Discuss DAS management considerations
Discuss DAS challenges

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What is DAS?
External Direct Connect
Internal Direct Connect
Uses block level protocol for data access

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DAS Benefits
Ideal for local data provisioning
Quick deployment for small environments
Simple to deploy
Low capital expense
Low complexity

6. DAS Connectivity Options
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DAS Connectivity Options
ATA (IDE) and SATA
Primarily for internal bus
SCSI
Parallel (primarily for internal bus)
Serial (external bus) FC
High speed network technology
Buss and Tag
Primarily for external mainframe
Precursor to ESCON and FICON

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DAS Management
Internal
Host provides:
Disk partitioning (Volume management)
File system layout
Direct Attached Storage managed individually through the server and the OS
External
Array based management
Lower TCO (Total Cost of Ownership) for managing data and storage Infrastructure

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DAS Challenges
Scalability is limited
Number of connectivity ports to hosts
Difficulty to add more capacity
Limited bandwidth
Distance limitations
Downtime required for maintenance with internal DAS
Limited ability to share resources
Array front-end port
Unused resources cannot be easily re-allocated
Resulting in islands of over and under utilized storage pools

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Lesson Summary
Key points covered in this lesson:
Internal and External DAS
DAS Benefit
DAS Management Options
DAS Limitations

10. Lesson: Introduction to SCSI
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Lesson: Introduction to SCSI
Upon completion of this module, you will be able to:
Describe SCSI-3 architecture
Discuss SCSI device models with different port configurations
Describe SCSI Addressing

11. Evolution of Parallel SCSI
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Evolution of Parallel SCSI
Developed by Shugart Associates & named as SASI (Shugart Associates System Interface)
ANSI acknowledged SCSI as an industry standard
SCSI versions
SCSI–1
Defined cable length, signaling characteristics, commands & transfer modes
Used 8-bit narrow bus with maximum data transfer rate of 5 MB/s
SCSI–2
Defined Common Command Set (CCS) to address non-standard implementation of the original SCSI
Improved performance, reliability, and added additional features
SCSI–3
Latest version of SCSI
Comprised different but related standards, rather than one large document

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SCSI–3 Architecture
SCSI Primary
Commands
SCSI Specific
Physical Layer
SCSI-3 Command Protocol
Transport Layer
Common
Access
Method
SCSI Architectural Model
SCSI-3
Protocol
Fibre Channel
Serial Bus
Generic
Packetized
Parallel
Interface
IEEE
Fibre
Channel
SCSI command protocol
Primary commands common to all devices
Transport layer protocol
Standard rules for device communication and information sharing
Physical layer interconnect
Interface details such as electrical signaling methods and data transfer modes

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SCSI Device Model
SCSI communication involves:
SCSI initiator device
Issues commands to SCSI target devices
Example: SCSI host adaptor
SCSI target device
Executes commands issued by initiators
Examples: SCSI peripheral devices
SCSI
Initiator Device
Target Device
Application
Client
Logical Unit
Device Service
Response
Request
Task Management
Device
Server
Task
Manager

14. SCSI Device Model (Cont.)
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SCSI Device Model (Cont.)
Device requests uses Command Descriptor Block (CDB)
8 bit structure
Contain operation code, command specific parameter and control parameter
SCSI Ports
SCSI device may contain initiator port, target port, target/initiator port
Based on the port combination, a SCSI device can be classified as an initiator model, a target model, a target model with multiple ports or a combined model (target/initiator model). Example:
Target/initiator device contain target/initiator port and can switch orientations depending on the role it plays while participating in an I/O operation
To cater to service requests from multiple devices, a SCSI device may also have multiple ports (e.g. target model with multiple ports)

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SCSI Addressing
LUNs
Initiator
Target
Initiator ID
Target ID
LUN
Initiator ID - a number from 0 to 15 with the most common value being 7.
Target ID - a number from 0 to 15
LUN - a number that specifies a device addressable through a target.

16. SCSI Addressing Example
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SCSI Addressing Example
Target (Front-end port)
Target – t0
LUN d0
Port
Storage Volumes
Port d1
Port d2
Port
Port
Host
Initiator (HBA)
Controller – c0
Storage Array
Host Addressing:
Storage Volume 1 - c0t0d0 Storage Volume 2 - c0t0d1 Storage Volume 3 - c0t0d2
Initiator ID
Target ID
LUN c0 t0 d0

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Lesson Summary
Key points covered in this lesson:
SCSI – 3 Architecture
SCSI device model
SCSI addressing
Additional Task
Refer www,t10.org for updated
information on SCSI

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Chapter Summary
Key points covered in this chapter:
DAS can be internal or external
DAS challenges
SCSI – 3 architecture
SCSI addressing

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Check Your Knowledge
Give an example of when DAS is a good solution?
List SCSI Device Models with Different Port Configurations?
How many devices SCSI can support?
Which SCSI ID has highest priority?
Additional Task
Research on Serial Attached SCSI (SAS)

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