Pass DSDSDSDSDSDSDSDSSDSSDSDSSDDSDSDSDS .pdf

Sharath D s Pass

1. © 2009 EMC Corporation. All rights reserved.
EMC Proven Professional

The #1 Certification Program in the information storage
and management industry

Direct Attached Storage
and
Introduction to SCSI
Chapter 5
Section 2 : Storage Networking Technologies and Virtualization

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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

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5.1 What is DAS?
Uses block level protocol for data access
Internal DAS
External DAS

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

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Limitations
DAS does not scale well- limited number of ports.
Do not make optimal use of resources – limited
front end ports.
Limited bandwidth restricts I/O processing
capability – leads to ripple effect.
Unused resources cannot be re-allocated, leading
to over-utilized and under-utilized storage pools.

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5.3 Disk Drive Interfaces
DAS uses predefined protocols.
These are implemented on HDD Controller.
1.IDE/ATA
2.SATA
Primarily for internal bus
3.Parallel SCSI (primarily for internal bus)
 Serial SCSI (external bus)
4.FC
1.High speed network technology

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IDE/ ATA
Integrated Device Electronics/ Advanced Technology
Attachment.
IDE specifies how controllers of devices connect to
motherboard.
ATA connects devices such as floppy drives, CD-ROM.
ATA/ATAPI, EIDE, ATA-2, Fast ATA, Ultra DMA…

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SATA
Serial version of IDE/ATA.
Point-to-point connectivity up to 1 meter and 150MB/s
data transfer speed. Enhancement led to 600MB/s speed.
LVDS: Low-voltage differential signaling provide high
speed over low cost. 250mV for high speed transfer.
7-pin connector and thin cable.
Hot-pluggable.
Single drive connectivity eliminates performance
problems.

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Parallel SCSI
Oldest form of software interface used by hosts.
SCSI often used to connect to HDDs and tapes to host.
Scanners and printers also connected.
SCSI-1 - 5MB/s, SCSI Ultra 320 – 320 MB/s.

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5.4 Introduction to Parallel SCSI
Developed by Shugart Associates & NCR developers,
named it as SASI (Shugart Associates System Interface)
ANSI acknowledged SCSI as an industry standard.
SCSI offers improved performance, expandability and
compatibility options.
Suitable for high-end computers.
High cost limits popularity.

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5.4.1 Evolution of SCSI

SCSI–1
Defined cable length, signaling characteristics, commands
& transfer modes
Used 8-bit narrow bus with maximum data transfer rate
5MB/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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5.4.2 SCSI Interfaces

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5.4.3 SCSI–3 Architecture
SCSI Primary
Commands
SCSI Specific
Commands
Physical Layer
SCSI-3 Command Protocol
Transport Layer
Common
Access
Method
SCSI Architectural Model
SCSI-3
Protocol
Fibre Channel
Protocol
Serial Bus
Protocol
Generic
Packetized
Protocol
SCSI-3
Parallel
Interface
IEEE
Serial Bus
Fibre
Channel

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3 major components of SCSI architectural model
1.SCSI command protocol
Primary commands common to all devices
Decive – specific commands.

2.Transport layer protocol
Standard rules for device communication and information sharing

3.Physical layer interconnects
Interface details such as electrical signaling methods and data
transfer modes

Common access methods – ANSI software interfaces for
SCSI devices

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

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SCSI Client-Server Model (contd…)
Device requests uses Command Descriptor Block (CDB)
8 bit structure
Contain operation code, command specific parameter and
control parameter
SCSI ID.
A spicific number to identify SCSI devices.
Narrow SCSI – bus width=8, numbered 0 to 7, Wide SCSI
numbered 0 to 15.
IDs set device priorities. Narrow bus - 7 highest priority & 0 has
least.
Wide bus – 8 to 15 has highest priorities than 0 to 7.
Wide bus has least priority than Narrow bus SCSI.

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SCSI Ports

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SCSI Communication Model

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3 layers in Communication Model
1.SCSI application layer (SAL): contains both client and server
applications that initiate and process SCSI I/O operations
using SCSI application protocol.
2.SCSI transport protocol layer (STPL): contains the services
and protocols that allow communication between initiator
and targets.
3.Interconnect layer: facilitates data transfer between
initiator and targets.
Also known as the service delivery subsystem - services, signalling
mechanisms, and interconnects for data transfer.

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5.4.4 Parallel SCSI Addressing

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

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SCSI Initiator – Target Communication
Initiator ID Target ID LUN
c0 t0 d0
Port
Port
Port
Port
Port
Host
Storage Array
Target (Front-end port)
Target – t0

Initiator (HBA)
Controller – c0
d0
d1
d2
Storage
Volumes
Host Addressing:
Storage Volume 1 - c0 t0 d0
Storage Volume 2 - c0 t0 d1
Storage Volume 3 - c0 t0 d2
LUN
LUN
LUN

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

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