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Chapter 18 - Recovery and fault tolerance. This chapter discusses recovery and fault tolerance techniques used in a distributed operating system. Resiliency, which is a technique for minimizing the impact of a fault, is also discussed.
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Chapter 18: Recovery and Fault Tolerance Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 1 Copyright © 2008 Introduction • A fault may have several consequences – It may damage state of some data or processes, leading to * Malfunctioning of a server * Non-availability of resources and services * Disruption of system operation – Its consequences can be avoided using three approaches * Recovery Some data or processes are rolled back to previous states * Fault tolerance Provides un-interrupted operation of a system despite faults * Resiliency Reduces cost of re-execution following a fault – Recovery is the generic term for all three approaches Chapter 18: Recovery and Fault Tolerance Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 2 Copyright © 2008 Faults and failures • A fault damages the state of a system – We say it causes an error in state of a system – It leads to unexpected behavior, which we call a failure – Recovery restores the system to an error-free state Chapter 18: Recovery and Fault Tolerance Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 3 Copyright © 2008 Recovery after a fault • System operation is initiated in state S0 at time 0 ’ ; a failure is detected at time t when state is S • A fault occurs at time t i i RecoveryputsthesystemintoanewstateSnew,whichiserrorưfree Chapter18:Recoveryand FaultTolerance Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:4 Copyrightâ2008 Classes of faults • A fault model describes properties of a fault – System fault is a system crash caused by a power outage or component fault * Amnesia fault The system loses its state completely * Fail-stop fault The system stops operating when a fault occurs This property permits an error in system state to be corrected * Byzantine fault A process suffering this fault behaves maliciously * Storage fault Bad block on a store medium Chapter18:Recoveryand FaultTolerance Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:5 Copyrightâ2008 Recovery techniques Four techniques of recovery – Data recovery * State of data or file is restored to that in its latest back-up – Process recovery * A checkpoint is a recording of the state of a system * At a fault, a process is rolled back to the state in some checkpoint – Fault tolerance * The error in state is corrected without interruption of system operation – Resiliency * Some results produced by a computation before a fault occurred are used during recovery Chapter 18: Recovery and Fault Tolerance Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 6 Copyright © 2008 Recovery techniques • Two classes of recovery techniques – Backward recovery * The state of an entity or application is reset to a prior state Simple to implement However, state recording should be both feasible and practical – Forward recovery * The erroneous state of an entity or application is repaired Chapter 18: Recovery and Fault Tolerance Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 7 Copyright © 2008 Byzantine faults and agreement protocols • A faulty entity may perform wrong actions; a fault tolerance technique is used to avoid their consequences – The Byzantine agreement problem: A group of processes must agree on a value * Initiator broadcasts the value to all other processes * On receiving the value, each process broadcasts it to all others A non-faulty process broadcasts the received value A faulty process may broadcast a different value * All non-faulty processes must conclude the correct value * Lamport’s protocol involves m+1 rounds of messages, where number of faulty processes does not exceed m * Impossibility result: a group of processes involving one faulty process cannot reach agreement Chapter 18: Recovery and Fault Tolerance Dhamdhere: Operating Systems— AConceptưBasedApproach,2ed SlideNo:8 Copyrightâ2008 Recovery schemes A recovery scheme consists of two algorithms: – Checkpointing algorithm * Decides when and how to create checkpoints of processes * Cij denotes the j’th checkpoint of process Pi * State (Cij) denotes the state of process Pi in checkpoint Cij – Recovery algorithm * When a failure is noticed, it decides which checkpoint a process should be rolled back to Chapter 18: Recovery and Fault Tolerance Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:9 Copyrightâ2008 Checkpoints and recovery Attimetf,failureoccursinnodeN3,whereprocessP3isinoperation RollingbackP3toC32wouldbeinconsistentbecausesendingof messagem3byP3wouldbenullified,whereasP2hasreceivedm3 Chapter18:Recoveryand FaultTolerance Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:10 Copyrightâ2008 Orphan messages Definition – A message mk sent by process Pi to Pj is an orphan in the new state of a system during recovery if the new state * Records mk as received by Pj * Does not record mk as sent by Pi • If rolling back of Pi makes mk an orphan, then Pj must be rolled back to a state before it had received mk Chapter 18: Recovery and Fault Tolerance Dhamdhere: Operating Systems— AConceptưBasedApproach,2ed SlideNo:11 Copyrightâ2008 Fault tolerance techniques A fault tolerance technique ensures that – Either a fault cannot cause an error – Or the error can be removed easily * It is typically achieved through data recovery • We discuss tolerance of storage faults – Its two aspects are * Fault tolerance for replicated data Data should remain available; processes should see correct values * Fault tolerance for distributed data Different parts of data should not become mutually inconsistent Chapter 18: Recovery and Fault Tolerance Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 12 Copyright © 2008 Logs, forward recovery and backward recovery • A log is a record of activities in a process Its contents are used for fault tolerance and recovery – Do log * Records those actions that should be performed to ensure correctness of state of an entity or system – Undo log * Records actions that should be undone to remove an error in state if a fault occurs – Write ahead logging * A log record is written before an action is taken * Ensures that the log contains correct entries when faults occur – Operation log contains a list of actions – Value log contains a list of values of data Chapter 18: Recovery and FaultTolerance Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:13 Copyrightâ2008 Reliable data replication A quorum is a specified number of copies of data D that should be accessed by a process – Qr: quorum for reading, Qw: quorum for writing – How to ensure that D can be updated by only one process but can be read by many, and that a process would always read a latest value of D? * x Qw > n * * Qr + Qw > n Use time-stamps to identify the latest value – How to tolerate up to k failures? * Qr = k + * * Qw = n – k n>2xk Chapter18:Recoveryand FaultTolerance Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:14 Copyrightâ2008 Reliable distributed data Inability to update some parts of distributed data due to link or node faults would cause data inconsistency – A distributed transaction is used to avoid it – The two phase commit protocol (also called PC protocol) is used to tolerate link or node faults * It ensures one of the following conditions All sites containing parts of distributed data can perform updates reliably None of the parts of the distributed data is updated Chapter 18: Recovery and Fault Tolerance Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 15 Copyright © 2008 Two phase commit protocol – Phase I Actions of transaction coordinator: a Write a ‘Prepare Ti’ record in the log b Set a time-out and send a ‘Prepare Ti’ message to all nodes participating in the transaction Actions of a participating node: a If it is ready to commit, write updates in stable storage and a ‘Prepared Ti’ record in the log, and send a ‘Prepared Ti’ message to coordinator b Otherwise, write an ‘Abandoned Ti’ record in the log and send an ‘Abandoned Ti’ message to coordinator Chapter 18: Recovery and Fault Tolerance Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 16 Copyright © 2008 Two phase commit protocol – Phase II Actions of transaction coordinator: a If it receives a ‘Prepared’ reply from all nodes before time-out occurs, write a ‘Commit Ti’ record in the log and send ‘Commit Ti’ messages to all nodes b Otherwise, write an ‘Abort Ti’ record in the log and send ‘Abort Ti’ messages to all nodes c Wait for an acknowledgment from each node and write a ‘Complete Ti’ message in the log Actions of a participating node: Depending on the coordinator’s message, a Write a ‘Commit Ti’ record in the log and perform commit processing b Write an ‘Abort Ti’ record in the log and abandon updates of Ti Chapter 18: Recovery and Fault Tolerance Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed SlideNo:17 Copyrightâ2008 Resiliency Consider a distributed computation Parts of the computation are executed in different nodes * These parts communicate through interprocess messages – Faults can arise during its operation * A node containing a part of the computation or its data fails * A link fails – However, failures are partial * A node or link failure may affect only some part of the computation * Resiliency techniques exploit this fact to reduce the recovery cost Some of the results computed before the failure are re-used Only some parts of the computation are re-executed Chapter18:Recoveryand FaultTolerance Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:18 Copyrightâ2008 Nested transaction A nested transaction Tik is a part of an atomic transaction Ti – It commits only if its parent transaction Ti commits – It is implemented as follows * When Tik completes, it is said to have reached a tentative commit * When Ti wishes to commit, it checks whether all nested transactions have reached a tentative commit and can participate in commit processing It is implemented using a 2PC protocol Chapter 18: Recovery and Fault Tolerance Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 19 Copyright © 2008 Nested transaction • Resiliency is implemented as follows: – If a nested transaction Tik does not respond to a ‘Prepare’ message, the coordinator can retry Tik in the same node or in some other node * If Tik had reached tentative commit and its node had failed when ‘Prepare’ message was sent * If the failed node recovers and the coordinator retries Tik in it * The results of Tik, computed before the failure, can be used Chapter 18: Recovery and Fault Tolerance Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 20 Copyright © 2008 ... parts of the computation are re-executed Chapter1 8:Recoveryand FaultTolerance Dhamdhere: OperatingSystems AConceptưBasedApproach,2ed SlideNo :18 Copyrightâ2008 Nested transaction A nested transaction... back of Pi makes mk an orphan, then Pj must be rolled back to a state before it had received mk Chapter 18: Recovery and Fault Tolerance Dhamdhere: Operating Systems— AConceptưBasedApproach,2ed... state of an entity or application is repaired Chapter 18: Recovery and Fault Tolerance Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 7 Copyright © 2008 Byzantine faults