Storage Networking Technologies
and Virtualization
Section 2
DAS and Introduction to SCSI
1
Section Objective
Upon completion of this section, you will be able to:
• Describe the elements, connectivity, and
management of:
–
–
–
–
Direct Attached Storage (DAS)
Network Attached Storage (NAS)
FC and IP Storage Area Networks (SAN)
Content Addressed Storage (CAS) and
• Discuss the benefits and challenges of each of the
storage models
• Describe various virtualization technologies
DAS and Introduction to SCSI
-2
Direct Attached Storage and
Introduction to SCSI
Chapter 5
DAS and Introduction to SCSI
3
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
DAS and Introduction to SCSI
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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
DAS and Introduction to SCSI
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What is DAS?6
Internal Direct Connect
• DAS
Uses
block level protocol for data access
and Introduction to SCSI
-6
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 and Introduction to SCSI
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DAS Connectivity Options
• ATA(133 MB/sec) (IDE) and SATA(150 MB/sec too 600)
– Primarily for internal bus
(40pin-disk to MB,34pin-drive to MB)
(latest ATA Ultra DMA/133)
• 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 and Introduction to SCSI
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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 for managing data and storage
Infrastructure
DAS and Introduction to SCSI
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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
DAS and Introduction to SCSI
10
Lesson Summary
Key points covered in this lesson:
• Internal and External DAS
• DAS Benefit
• DAS Management Options
• DAS Limitations
DAS and Introduction to SCSI
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Lesson: Introduction to SCSI
Upon completion of this lesson, you will be able
to:
• Describe SCSI-3 architecture
• Discuss SCSI device models with different port
configurations
• Describe SCSI Addressing
DAS and Introduction to SCSI
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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.
DAS and Introduction to SCSI
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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
•
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
DAS and Introduction to SCSI
SCSI Device Model
SCSI communication involves:
• SCSI initiator device
•
SCSI target device
– Issues commands to SCSI target
devices
– Example: SCSI host adaptor
– Executes commands issued by
initiators
– Examples: SCSI peripheral devices
•
SCSI
Initiator Device
Device requests contain Command
Descriptor Block (CDB)
SCSI
Target Device
Device Service
Request
Application
Client
Device Service
Response
Task Management
Request
Task Management
Response
DAS and Introduction to SCSI
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Logical Unit
Device
Server
Task
Manager
SCSI Device Model (Cont.)
• CDB (command descriptor Block) 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
DAS and Introduction to SCSI
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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.
DAS and Introduction to SCSI
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
DAS and Introduction to SCSI
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t0
LUN
d0
Cost
Expansion
Ease
ATA/IDE
Overall, IDE is
a much
cheaper
solution.
SCSI
When
compared
with IDE, SCSI
is often more
expensive to
implement
and support.
IDE/EIDE
SCSI is
allows 2 two capable of
devices per
supporting up
channel. Most to 7 or 15
computers
devices.
have 2
channels.
IDE is
commonly a
much easier
product to
setup than
SCSI.
Configuring
SCSI can be
more difficult
for most users
when
compared to
IDE.
DAS and Introduction to SCSI
19
Resources
ATA/IDE
SCSI
All motherboards
today have an
ATA/IDE interface
and unless
additional drives
are needed no
additional
resources need
to be taken.
Unlike IDE, SCSI
requires an
interface
expansion card in
most cases
(unless the
motherboard
already has it).
This means
additional
system resources
will be required.
DAS and Introduction to SCSI
20
Lesson Summary
Key points covered in this lesson:
• SCSI – 3 Architecture
• SCSI device model
• SCSI addressing
• Comparison of SCSI & IDE/ATA
DAS and Introduction to SCSI
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Chapter Summary
Key points covered in this chapter:
• 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
DAS and Introduction to SCSI
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
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?
DAS and Introduction to SCSI
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