implementation of concurrent access to file systems in usb devices
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Implementation of Concurrent Access to File Systems in USB Devices. GSPx 2004 Bogdan Vacaliuc. Overview. Advocacy Description of the problem Related Work Description of a solution Implementation Requirements Enhancements. Why do we need this?.

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Presentation Transcript
overview
Overview
  • Advocacy
  • Description of the problem
  • Related Work
  • Description of a solution
  • Implementation Requirements
  • Enhancements

2

why do we need this
Why do we need this?
  • Intelligent devices produce/consume media content.
  • MSDC is ubiquitous for USB devices.
  • Implementing concurrent access enables:
    • Command/Control via file sharing.
    • Firmware Upgrades.
    • Exchange of data between host and device.

… All without the need for OS-specific drivers …

3

the concurrency problem
The Concurrency Problem
  • Host MSDC implicitly caches file system data and metadata.
  • Device modifies data and metadata.
  • Without synchronization, host modifications can supercede device modifications.

4

related work
Related Work
  • Distributed File Systems
    • Granularity of File Object.
    • NFS, AFS
    • Sprite, SpritelyNFS, NQNFS
  • Shared Block Devices
    • Granularity of Logical Block on Media
    • GPFS, GFS/OpenGFS
    • NBD, Shared SCSI

6

a solution
A Solution
  • Primary Requirements
    • It must be simple
    • It must operate with existing host MSDC drivers
  • Secondary Requirements
    • FAT File System
    • Shared Block Device approach

7

a solution cont
A Solution (cont.)
  • Establish epochs of exclusive access to metadata on the device
  • Define implicit criteria for obtaining exclusive access to the media by the host
  • Define explicit criteria for releasing exclusive access by the host
  • Implement fine-grain locking on accesses made by the device.

8

use cases
Use Cases
  • Client creates new file
    • Metadata is modified
  • Client modifies file contents
    • File data is modified
  • Client extends file
    • Both file and metadata are modified

10

usb msdc principles
USB/MSDC Principles
  • Command Block Wrapper (CBW)
  • Command Status Word (CSW)
  • Identify device as “removable media”
  • Use Unit Attention condition to notify host of changed data

12

implementation requirements device side
Implementation Requirements – Device Side
  • Identify as ‘removable media’ device
  • Use Unit Attention condition to notify host of media content change
  • Implement a structured block device and cache interface
  • Implement file system locking which is exposed to the USB driver

13

implementation requirements host side
Implementation Requirements – Host Side
  • Implement ‘eject media’ operation when file work is complete.
  • Can be user-driven (explicit drive letter ‘eject’)
  • Can be software-driven (close files and unmount/eject)
  • Behavioral differences between OS’es

14

summary
Summary
  • Correct operation under concurrent access conditions is obtained by:
    • Implicit locking based on USB media access
    • Explicit release via media eject operation
    • Use of Unit Attention condition
  • Features
    • No new driver added to host operating system
    • Minimal requirements on device (best practices)

16

enhancements and future work
Enhancements and Future Work
  • Would like to remove explicit eject requirement on host
    • Use SCSI_RESERVE commands to manage locking epoch begin and end
    • Requires Installable File System (IFS) driver

17

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