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© 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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 3
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 4
What is DAS?
Uses block level protocol for data access
Internal Direct Connect External Direct Connect
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 5
DAS Benefits
Ideal for local data provisioning
Quick deployment for small environments
Simple to deploy
Reliability
Low capital expense
Low complexity
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 6
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 7
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 8
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 9
Lesson Summary
Key points covered in this lesson:
Internal and External DAS
DAS Benefit
DAS Management Options
DAS Limitations
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 10
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 11
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.
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 12
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 13
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 14
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 15
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 16
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 17
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 18
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 19
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 20
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 21
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 22
Lesson Summary
Key points covered in this lesson:
SCSI – 3 Architecture
SCSI device model
SCSI addressing
SCSI command model
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 23
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
© 2009 EMC Corporation. All rights reserved. DAS and Introduction to SCSI - 24
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?