© 2009 EMC Corporation. All rights reserved. Direct Attached Storage and Introduction to SCSI...

© 2009 EMC Corporation. All rights reserved.

Direct Attached Storage and Introduction to SCSIDirect Attached Storage and Introduction to SCSI

Module 2.1

© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 2

DAS and Introduction to SCSI

Upon completion of this module, 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


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


What is DAS?

Uses block level protocol for data access

Internal Direct Connect External Direct Connect


DAS Benefits

Ideal for local data provisioning

Quick deployment for small environments

Simple to deploy

Reliability

Low capital expense

Low complexity


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


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 for managing data and storage Infrastructure


DAS Challenges

Scalability is limited– Number of connectivity ports to hosts

– Number of addressable disks

– Distance limitations

Downtime required for maintenance with internal DAS

Limited ability to share resources– Array front-end port, storage space

– Resulting in islands of over and under utilized storage pools


Lesson Summary

Key points covered in this lesson:

Internal and External DAS

DAS Benefit

DAS Management Options

DAS Limitations


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


Evolution of Parallel SCSI

Developed by Shugart Associates & named as SASI

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) Improved performance, reliability, and added additional features

– SCSI–3 Latest version of SCSI, Comprised different but related standards, rather than one large

document.


SCSI Interfaces (Parallel)

Interface Standard Width ClockMAX Throughput

MAX Devices

SCSI-1 SCSI-1 (1986) 8 5 MHz 5 MB/s 8 Fast SCSI SCSI-2 (1994) 8 10 MHz 10 MB/s 8

Fast-Wide SCSI

SCSI-2;SCSI-3 SPI (1996)

16 10 MHz 20 MB/s 16

Ultra SCSI SCSI-3 SPI 8 20 MHz 20 MB/s 8 Ultra Wide SCSI

SCSI-3 SPI 16 20 MHz 40 MB/s 16

Ultra2 SCSISCSI-3 SPI-2 (1997)

8 40 MHz 40 MB/s 8

Ultra2 Wide SCSI

SCSI-3 SPI-2 16 40 MHz 80 MB/s 16

Ultra3 SCSISCSI-3 SPI-3 (1999)

16 40 MHz DDR 160 MB/s 16

Ultra-320 SCSI

SCSI-3(2002) 16 80 MHz DDR 320 MB/s 16

Ultra-640 SCSI

SCSI-3(2003) 16 160 MHz DDR 640 MB/s 16


SCSI–3 Architecture

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

SCSI Primary Commands

SCSI Specific Commands

Physical Layer

SCSI-3 Command Protocol

Transport LayerCommon Access Method

SCSI Architectural Model

SCSI-3 Protocol

Fibre ChannelProtocol

Serial BusProtocol

GenericPacketized

Protocol

SCSI-3 Parallel

Interface

IEEE Serial Bus

Fibre Channel


SCSIInitiator Device

SCSITarget Device

ApplicationClient

Logical UnitDevice Service

Response

Device Service Request

Task Management Request

Task Management Response

DeviceServer

TaskManager

SCSI target device– Executes commands issued by

initiators

– Examples: SCSI peripheral devices

Device requests contain Command Descriptor Block (CDB)

SCSI Device Model

SCSI communication involves:

SCSI initiator device– Issues commands to SCSI target

devices

– Example: SCSI host adaptor


SCSI Device Model (Cont.)

CDB structure– 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 port combination device is classified

– For 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


SCSI Addressing

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.

Initiator ID Target ID LUN

Target Initiator LUNs


SCSI Addressing Example

Initiator ID Target ID LUN

c0 t0 d0

Port

Port

Port

Port

Port

HostStorage Array

Target (Front-end port)Target – t0

Initiator (HBA)Controller – c0

d0

d1

d2

Storage Volumes

Host Addressing:

Storage Volume 1 - c0t0d0 Storage Volume 2 - c0t0d1 Storage Volume 3 - c0t0d2

LUN

LUN

LUN


SCSI Command Model

The SCSI command model is defined with the CDB.

7 6 5 4 3 2 1 0 0 Operation Code 1

Command Specific-Parameters n–1

n Control

Bit

Byte


Operation Code

The operation code consists of a group and command code fields.

Bit 7 6 5 4 3 2 1 0

Group Code Command Code

Group Code Command-Specific Parameters

0 6 bytes

1 and 2 10 bytes

3 Reserved

4 16 bytes

5 12 bytes

6 and 7 Vendor specific


Common SCSI CommandsCommand DescriptionREAD Reads data from a deviceWRITE Writes data to a device

TEST UNIT READYQueries the device to check whether it is ready for data transfer

INQUIRYReturns basic information, which is also used to ping the device

REPORT LUNS List the logical unit numbersSEND AND RECEIVEDIAGNOSTIC RESULTS

Runs a simple self-test or a specialized test defined in a diagnostic page

FORMAT UNITSets all sectors to all zeroes and allocates logical blocks, avoiding defective sectors

LOG SENSE Returns current information from log pages

LOG SELECTUsed to modify data in the log pages of a SCSI target device

MODE SENSEReturns current device parameters from mode pages

MODE SELECT Sets device parameters on a mode page


Control Field & Status

Bit 7 6 5 4 3 2 1 0

Vendor Specific Reserved NACA Obsolete Link

Status Byte Codes Status 0h GOOD 2h CHECK CONDITION 4h CONDITION MET 8h BUSY 10h INTERMEDIATE 14h INTERMEDIATE-CONDITION MET 18h RESERVATION CONFLICT 22h COMMAND TERMINATED 28h TASK SET FULL 30h ACA ACTIVE All other codes Reserved


Lesson Summary

Key points covered in this lesson:

SCSI – 3 Architecture

SCSI device model

SCSI addressing

SCSI command model


Module Summary

Key points covered in this module:

DAS can be internal or external

Multiple hosts cannot share same storage ports

DAS is made up of a CPU, connectivity, and storage devices

DAS connectivity uses block-level access protocols

SCSI – 3 architecture

Parallel SCSI addressing

SCSI command model


Check Your Knowledge

What are the physical elements of DAS?

Give an example of when DAS is a good solution.

Describe internal DAS connectivity.

Describe external DAS connectivity.

List SCSI Device Models with Different Port Configurations.

How many devices SCSI can support?

Which SCSI ID has highest priority?

© 2009 EMC Corporation. All rights reserved. Direct Attached Storage and Introduction to SCSI...

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