Raid-II: A High-Bandwidth Network File Server

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Raid-II A High-Bandwidth Network File Server

• Ann L. Drapeau, Ken W. Shirriff, John H. Hartman,
  Ethan L. Miller, Srinivasan Seshan, Randy H.
  Katz, Ken Lutz, David A. Patterson, Edward K.
  Lee, Peter M. Chen, Garth A. Gibson

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Introduction

• Why do we need RAID-II?
• Many new applications require larger amounts of
  data.
• Computer Aided Design
• Databases
• Multi-media
• Scientific Simulations
• Raid-I is unable to provide the needed I/O
  bandwidth.

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The Need For Speed

• RAID-I
• Redundant Arrays of Inexpensive Disks
• Performs well when processing small random I/O
  requests.
• Inadequate for high-bandwidth I/O requests.
• Coping data between kernel DMA buffers and
  buffers in user space saturate the memory system
  when I/O bandwidth reaches 2.3 mb/s.
• Data must go through the CPUs virtual addressed
  cache and cache flushes slow the performance.
• Bandwidth restricted by the VME system bus (9
  mb/s).

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Raid-II

• RAID-II contains two data paths
• High-bandwidth (HIPPI)
• Handles large data transfers.
• Low-bandwidth (Ethernet)
• Handles metadata and small data transfers.
• LFS Log Structured File System

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High-Bandwidth Data Path

• XBUS
• Allows large data requests to bypass the server
  workstation.
• Connects the disks directly to the high speed
  HIPPI network.
• The HIPPI network is the high speed network with
  a transfer rate of 100MB/s.
• It takes about 1.1 milliseconds to transfer a
  HIPPI packet. (Due to setup of HIPPI XBUS
  control registers)

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Low-Bandwidth Data Path

• Uses Ethernet capable of 10 Mb/s 1.25 MB/s.
• Used to transfer the metadata and small data
  requests.
• It takes about 0.5 milliseconds to transfer a
  packet on Ethernet. (HIPPI takes 1.1 ms)

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RAID-II
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XBUS Controller Board
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Raid-II file system

• LFS Log Structured File System
• Writes all file data and metadata to an append
  only log file.
• This minimizes the number of small writes to the
  disk, by buffering the writes and then writing a
  single large sequential file to the disk.
• Also provides fast crash recovery.

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Raid-II file system

• Modified LFS
• Must be able to separate requests among the high
  and low bandwidth data paths.
• Manage the memories on the host workstation and
  the XBUS board.
• Cache coherency

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Conclusions

• RAID-II allows data to be transferred between
  disks and the HIPPI network without going through
  the low-bandwidth memory by using the XBUS
  controller cards.
• RAID-II uses LFS to buffers small writes into
  large requests to avoid wasting disk bandwidth.

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Questions?

• What was the purpose of designing the XBUS
  controller board?
• Describe the two network connections used and
  their purposes.
• What additional ideas could be applied to RAID-II
  to improve performance?