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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Implementation of Concurrent Access to File Systems in USB Devices

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Implementation of concurrent access to file systems in usb devices

Implementation of Concurrent Access to File Systems in USB Devices

GSPx 2004

Bogdan Vacaliuc


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


The system model

The System Model

5


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


Serializing access

Serializing Access

9


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


Timeline of typical session

Timeline of Typical Session

11


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


The system model1

The System Model

15


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


Thank you

Thank You

18


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