MovementBased Checkpointing and Logging for Recovery in Mobile Computing Systems

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Movement-Based Check-pointing and Logging for
Recovery in Mobile Computing Systems

• Sapna E. George, Ing-Ray Chen, Ying Jin
• Dept. of Computer Science
• Virginia Polytechnic and State University

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Outline

• Background
• Problem Definition Failure Recovery in the
  Mobile Computing Environment
• Proposed Solution Movement-Based Check-pointing
  and Logging
• Performance Analysis
• Analytic Model of the System
• Analysis Results and Conclusions
• Future Work

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Background
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Mobile Computing

• Advances in wireless networking and portable
  device technologies are revolutionizing computing
• Mobile Computing A type of distributed
  computing
• Involves hosts that may be mobile
• Host network connectivity maintained through
  wireless communications

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Fault-tolerance in Distributed systems

• Check-pointing, Logging, Rollback recovery
• Check-pointing ? failure-free operations
• Save system state to stable storage
• This snapshot is called a checkpoint
• Logging ? failure-free operations
• All non-deterministic events and the information
  necessary to replay these events are logged to
  the stable storage
• In addition to checkpoints

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Fault-tolerance in Distributed systems

• Failure Recovery
• Failed process rolls back to the latest
  checkpoint
• Replays all the logged events in their original
  order
• Recreates pre-failure state independently

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Problem Definition

• Failure Recovery in the Mobile Computing
  Environment

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Effects of Properties of MC Env.

• Mobility of hosts
• If checkpointing requires coordination, the MH
  must be searched and located first before control
  messages can be delivered this increases
  communication delay
• Data related to recovery, such as checkpoints and
  logs, may be distributed over many MSS a
  mechanism is required for efficient storage,
  retrieval and management of this dispersed
  information

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Effects of Properties of MC Env.

• Low bandwidth and unreliable network connectivity
  
• A recovery mechanism that requires a large number
  of messages or large size of messages imposes
  undue burden on the wireless resources and
  increases the cost of providing fault tolerance.

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Effects of Properties of MC Env.

• Limited battery life of host devices
• Communication is energy intensive.
• Recovery mechanism must keep communication (the
  number of messages and the size of messages) to a
  minimum.

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Effects of Properties of MC Env.

• Lack of stable storage on host devices
• Devices are vulnerable to physical damage
• Devices are small and are equipped with limited
  memory
• MHs disk cannot reliably function as the stable
  storage required to store recovery information.

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Effects of Properties of MC Env.

• Different types of failures
• Voluntary disconnection and hardware failure must
  be handled differently
• A disconnected host may reconnect after a while
  and expect to resume operations
• A MH that is currently unreachable cannot be
  expected to participate in a checkpointing or
  recovery operation.
• A scheme that requires synchronization or
  coordination with other MHs would either block
  until the MH reconnected or would fail.

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The Problem

• Traditional recovery schemes suffer from many
  shortcomings when applied to the mobile computing
  environment.
• The failure-prone nature of the environment makes
  it essential to provide some form of explicit
  recovery mechanism.

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The Problem

• In general, application recovery mechanisms try
  to balance
• Recovery cost (failure-free operational cost)
• Recovery time
• Storage requirements for recovery related
  information

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The Problem

• Adaptations of traditional recovery schemes for
  the mobile computing environment
• Do not consider mobility in the selection of
  checkpointing interval
• Use periodic checkpointing
• Subsequently control the proliferation of
  recovery information using techniques that merge
  logs and move the information closer to the MH.

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Proposed Solution

• Movement-Based Check-pointing and Logging

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Assumed Mobile Computing System

• A set of mobile hosts (MHs)
• They maintain network connectivity through a
  wireless link to a static mobile support station
  (MSS)
• A MSS handles all communications to and from MHs
  within its area of influence known as a cell
• Each MSS is equipped with enough volume of stable
  storage to store the state and log information

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Assumed Mobile Computing System

• Interactions between the MH and the network
  infrastructure relevant to failure recovery
• Handoff Cell boundary crossing
• Disconnection For power conservation
• Reconnection Possibly in a cell different from
  the one in which it disconnected

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Assumed Mobile Computation

• A distributed computation ? a number of processes
  executing concurrently on multiple hosts.
• Process states
• Normal- executing application related
  computations, receiving user inputs or sending
  and receiving messages.
• Save - saves its state as a checkpoint to the
  stable storage
• Between checkpoints, the process also logs all
  events (Normal state)
• Recovery Loads checkpoints and applies logs

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Movement-Based Checkpointing and Logging

• Interval between checkpoints is governed by the
  number of handoffs experienced by the MH and is
  not fixed
• MH maintains a handoff counter which is
  incremented by 1 every time a handoff occurs.
• When the value of the counter becomes greater
  than a threshold M, a checkpoint is taken.
• In between checkpoints, all write events related
  to a MH is also logged to the local MSS.

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Movement-Based Checkpointing and Logging

• The threshold M is a configurable parameter.
  Depends on
• User mobility rate
• Network the failure rate
• Application log arrival rate

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Movement-Based Checkpointing and Logging

• Thus, depending on the variability in the MHs
  mobility, the time interval between successive
  checkpoints differs.
• Recovery MH recovers independently without
  coordination with other MHs
• Upon reconnection, MH informs local MSS.
• Local MSS contacts MSS with latest checkpoint
• Local MSS contacts all MSS storing logs
• All data transferred to local MSS via wired
  network and to MH via wireless link
• MH rolls back and applies logs

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Movement-Based Checkpointing and Logging

• The performance of this scheme depends on
  identifying the optimal movement threshold M per
  user and application.
• Checkpoints and logs remain within acceptable
  range of the MHs current location and eliminates
  the need for information consolidation.
• Ensures acceptable recovery time since M bounds
  the number of MSSs from which logs must be
  retrieved.

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Performance Analysis

• Analytic Model

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Stochastic Petri-Net (SPN) Model
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SPN Model Parameters
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SPN Model Parameters

• Parameter Tk- Checkpoint rate of the MH
• Parameter Ti- Recovery rate of the MH inverse
  of recovery time
• i - number of handoffs experienced by the MH
  since the last checkpoint and before failure.

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Analytic Model Recovery Time
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Analytic Model Recovery Time

• Treq_rec - Time spent on recovery information
  requests
• Nmss_logs Number of MSSs storing logs
• Dmss - average hop count between MSScp and MSSrec

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Analytic Model Recovery Time

• Tckp_tx - Time spent on transmitting the latest
  checkpoint to the MH

• Tlog_tx - Time spent on transmitting the logs to
  the MH

• Trec - Time spent to rollback to the last
  checkpoint and apply the logs

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Analytic Model Cost of Recovery

• Tr Average Recovery time per failure
• Fr Recovery probability
• Tc Cost of recovery

No. of checkpoints before failure
No. of logs before failure
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SPN Evaluation Parameters

• Size of a log entry - 50B
• Size of a checkpoint - 2000B
• Bandwidth of wired network-2Mbps
• Ratio of bandwidth of wireless to wired network
  (r) - 0.1
• Time required to apply a log entry (Telog) -
  0.0001s
• Time required to transmit a log entry through the
  wireless channel (Tlog_w) - 0.002s
• Time required to transmit a checkpoint through
  the wireless channel (Tckp_w) - 0.08s

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Performance Analysis

• Results and Conclusions

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Recovery Probability vs. Recovery Time
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Recovery Probability vs. Log Arrival Rate
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Recovery Probability vs. Failure Rate
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Recovery Probability Recovery Time vs. Movement
Threshold
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Determining Optimal Movement Threshold that
Minimizes Recovery Cost Per Failure
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Conclusion Proposed Scheme

• An efficient failure recovery scheme for mobile
  computing systems based on movement-based
  checkpointing and logging
• Movement-based checkpointing and logging scheme
  takes a checkpoint only after the mobile node has
  made M movements (mobility handoffs).
• The value of M is governed by the failure rate,
  log arrival rate, and the mobility rate of the
  application and MH.
• Identify the optimal movement threshold M, when
  given the failure, mobility and log arrival
  rates, to minimize the cost of recovery per
  failure.

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Conclusion Practical Application

• Build a table at configuration time covering
  possible parameter values of the mobility rate
  and failure rate of the MH and log arrival rate
  of the mobile applications, and listing the
  optimal M value that would minimize the recovery
  cost per failure.
• At runtime, based on the measured rates, the
  optimal M may be selected dynamically to minimize
  the recovery cost per failure.
• Optimal M selected must also satisfy the
  specified recovery probability when given an
  application deadline to recover from a failure.