Transcript DAS and Introduction to SCSI

Section 2 : Storage Networking Technologies and Virtualization
Direct Attached Storage and Introduction to SCSI
Chapter 5
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Chapter Objective
Upon completion of this chapter, you will be able to:
o Discuss the benefits and challenges of DAS
o Discuss DAS management options
o Discuss evolution of SCSI
o Describe SCSI – 3 architecture
o Discuss SCSI addressing and communication model
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Lesson: Direct Attached Storage
Upon completion of this lesson, you will be able to:
o Discuss the benefits of DAS
o Describe the elements of DAS
o Discuss DAS management considerations
o Discuss DAS challenges
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What is DAS?
Internal Direct Connect
o Uses block level protocol for data access
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External Direct Connect
DAS Benefits
o Ideal for local data provisioning
o Quick deployment for small environments
o Simple to deploy
o Low capital expense
o Low complexity
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DAS Connectivity Options
o ATA (IDE) and SATA
o Primarily for internal bus
o SCSI
o Parallel (primarily for internal bus)
o Serial (external bus)
o FC
o High speed network technology
o Buss and Tag
o Primarily for external mainframe
o Precursor to ESCON and FICON
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DAS Management
o Internal
o Host provides:
o Disk partitioning (Volume management)
o File system layout
o Direct Attached Storage managed individually through the server and the
OS
o External
o Array based management
o Lower TCO (Total Cost of Ownership) for managing data and storage
Infrastructure
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DAS Challenges
o Scalability is limited
o Number of connectivity ports to hosts
o Difficulty to add more capacity
o Limited bandwidth
o Distance limitations
o Downtime required for maintenance with internal DAS
o Limited ability to share resources
o Array front-end port
o Unused resources cannot be easily re-allocated
o Resulting in islands of over and under utilized storage pools
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Lesson Summary
Key points covered in this lesson:
o Internal and External DAS
o DAS Benefit
o DAS Management Options
o DAS Limitations
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Lesson: Introduction to SCSI
Upon completion of this module, you will be able to:
o Describe SCSI-3 architecture
o Discuss SCSI device models with different port configurations
o Describe SCSI Addressing
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Evolution of Parallel SCSI
o Developed by Shugart Associates & named as SASI (Shugart
Associates System Interface)
o ANSI acknowledged SCSI as an industry standard
o SCSI versions
o SCSI–1
o Defined cable length, signaling characteristics, commands & transfer modes
o Used 8-bit narrow bus with maximum data transfer rate of 5 MB/s
o SCSI–2
o Defined Common Command Set (CCS) to address non-standard
implementation of the original SCSI
o Improved performance, reliability, and added additional features
o SCSI–3
o Latest version of SCSI
o Comprised different but related standards, rather than one large document
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SCSI–3 Architecture
SCSI Architectural Model
SCSI-3 Command Protocol
SCSI Primary
Commands
Common
Access
Method
SCSI Specific
Commands
Transport Layer
SCSI-3
Protocol
Fibre Channel
Protocol
Serial Bus
Protocol
IEEE
Serial Bus
Fibre
Channel
Generic
Packetized
Protocol
Physical Layer
SCSI-3
Parallel
Interface
o SCSI command protocol
o Primary commands common to all devices
o Transport layer protocol
o Standard rules for device communication and information sharing
o Physical layer interconnect
o Interface details such as electrical signaling methods and data transfer modes
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SCSI Device Model
SCSI communication involves:
o SCSI target device
o SCSI initiator device
o Issues commands to SCSI target
devices
o Example: SCSI host adaptor
o Executes commands issued by
initiators
o Examples: SCSI peripheral devices
SCSI
Initiator Device
SCSI
Target Device
Device Service
Request
Application
Client
Device Service
Response
Task Management
Request
Task Management
Response
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Logical Unit
Device
Server
Task
Manager
SCSI Device Model (Cont.)
o Device requests uses Command Descriptor Block (CDB)
o 8 bit structure
o Contain operation code, command specific parameter and control
parameter
o SCSI Ports
o SCSI device may contain initiator port, target port, target/initiator port
o 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:
o Target/initiator device contain target/initiator port and can switch orientations
depending on the role it plays while participating in an I/O operation
o 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
o Initiator ID - a number from 0 to 15 with the most common value being 7.
o Target ID - a number from 0 to 15
o LUN - a number that specifies a device addressable through a target.
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SCSI Addressing Example
Target (Front-end port)
Target – t0
Port
Port
Port
LUN
d0
LUN
d1
LUN
d2
Storage
Volumes
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
c0
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t0
LUN
d0
Lesson Summary
Key points covered in this lesson:
o SCSI – 3 Architecture
o SCSI device model
o 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:
o DAS can be internal or external
o DAS challenges
o SCSI – 3 architecture
o SCSI addressing
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Check Your Knowledge
o Give an example of when DAS is a good solution?
o List SCSI Device Models with Different Port Configurations?
o How many devices SCSI can support?
o Which SCSI ID has highest priority?
Additional Task
Research on Serial Attached SCSI (SAS)
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