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Direct Attached Storage and Introduction to SCSI

Section 2 : Storage Networking Technologies and Virtualization. Direct Attached Storage and Introduction to SCSI. Chapter 5. Lecture 14. Introduction to Networked Storage: Evolution of networked storage. Chapter Objective. Upon completion of this chapter, you will be able to:

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Direct Attached Storage and Introduction to SCSI

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

  2. Lecture 14 • Introduction to Networked Storage: Evolution of networked storage

  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

  4. 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

  5. External Direct Connect What is DAS? • Uses block level protocol for data access Internal Direct Connect

  6. DAS Benefits • Ideal for local data provisioning • Quick deployment for small environments • Simple to deploy • Low capital expense • Low complexity

  7. 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

  8. 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 (Total Cost of Ownership) for managing data and storage Infrastructure

  9. DAS Challenges • Scalability is limited • Number of connectivity ports to hosts • Difficulty to add more capacity • Limited bandwidth • Distance limitations • Downtime required for maintenance with internal DAS • Limited ability to share resources • Array front-end port • Unused resources cannot be easily re-allocated • Resulting in islands of over and under utilized storage pools

  10. Lesson Summary Key points covered in this lesson: • Internal and External DAS • DAS Benefit • DAS Management Options • DAS Limitations

  11. 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

  12. Evolution of Parallel SCSI • Developed by Shugart Associates & named as SASI (Shugart Associates System Interface) • 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) 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

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

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

  15. SCSI Device Model (Cont.) • Device requests uses Command Descriptor Block (CDB) • 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 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: • 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 (e.g. target model with multiple ports)

  16. LUN Initiator ID Target ID SCSI Addressing LUNs Initiator Target • 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.

  17. LUN Initiator ID Target ID LUN LUN LUN SCSI Addressing Example Target (Front-end port) • Target – t0 d0 Port Storage Volumes Port d1 d2 Port Port Port Host Initiator (HBA) Storage Array Controller – c0 Host Addressing: Storage Volume 1 - c0t0d0 Storage Volume 2 - c0t0d1 Storage Volume 3 - c0t0d2 d0 t0 c0

  18. Lesson Summary Key points covered in this lesson: • SCSI – 3 Architecture • SCSI device model • SCSI addressing Additional Task Refer www,t10.org for updated information on SCSI

  19. Chapter Summary Key points covered in this chapter: • DAS can be internal or external • DAS challenges • SCSI – 3 architecture • SCSI addressing

  20. Check Your Knowledge  • Give an example of when DAS is a good solution? • List SCSI Device Models with Different Port Configurations? • How many devices SCSI can support? • Which SCSI ID has highest priority? Additional Task Research on Serial Attached SCSI (SAS)

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