Exploiting Weak Connectivity for Mobile File Access

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• Exploiting Weak Connectivity for Mobile File
  Access
• Lily B. Mummert, Maria R. Ebling, M.
  Satyanarayanan
• Presented by
• Shobith Alva

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Introduction

• Mobile clients will encounter a wide range of
  network characteristics.
• Weak Connectivity intermittent, low bandwidth,
  high latency, or high expense.
• Disconnected Operation represents an initial step
  toward supporting mobility

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Disconnected Operation

• Client continues to use data in its cache during
  temporary network failure.
• Limitations of Disconnected Operation
• Updates are not visible to other clients.
• Cache misses may impede progress.
• Updates are at risk due to theft, loss or damage.
• Update conflicts become more likely.
• Exhaustion of cache space is a concern.

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Goals

• Alleviate these limitations by exploiting weak
  connectivity.
• Modifications to Coda.

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CODA

• Is based on AFS (Andrew File System)?
• preserves model of security, scalability and unix
  compatibility.
• achieves high availability through the use of
  disconnected operation and server replication
• Venus
• hoarding
• emulating
• reintegrating

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Design Rationale

• Strategy
• incremental approach to extending coda
• Guiding principles for design
• dont punish strongly-connected clients
• dont make life worse than when disconnected
• do it in the background if you can
• when in doubt, seek user advice

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Modifications

• Transport protocol refinements
• Rapid cache validation
• trickle reintegration
• user assisted miss handling

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Transport Protocol

• Transport protocol refinements
• RPC2, SFTP
• fix to share keep alive information between rpc2
  sftp
• monitor network speed by RTT

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Rapid Cache Validation

• Rapid cache validation
• using callback
• raising the granularity of cache coherence via
  volume call back, individual object callback

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Trickle Integration

• Trickle reintegration
• reintegration vs. write disconnected
• using log optimization
• log opt. vs. promptness
• Aging window (1 hr)?

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