OCT Research Lab Edward C. Cheng

CS 415

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CS 415Operating Systems Principles

Logging and Recovery

Dr. Edward C. Cheng

OCT Research Lab Edward C. Cheng

CS 415

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Logging and Recovery

• Background

• System Model of the Logging System

• A Disintegration Model of Activity Recording

• 2-Phase Commit Protocol

• Rollback Recovery

OCT Research Lab Edward C. Cheng

CS 415

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System Model of Journaling

• Tightly coupled with the resource manager or application program

• Open Journal idea introduced in the 90’s separating journaling from the RM or AP

Fig 1. With OJS, multiple data streams can be open per AP/RM

Fig 2. Data streams are managed through life-cycle states

OCT Research Lab Edward C. Cheng

CS 415

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A Disintegration Model of Activity Recording

• Events that are not dependent on each other can have their activity records disintegrated thus provide various optimization opportunities

OCT Research Lab Edward C. Cheng

CS 415

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2-Phase Commit (2PC) Protocol

• A protocol to support Atomicity Behavior in distributive environment

• A coordinator (parent transaction) issues a request to commit to all subordinates (children transactions)

• Each subordinate writes a journal record to ensure ability to roll-forward and rollback in case of a crash

• Each subordinate responds to the parent with a ready-to-commit or abort

• If all subordinates are ready-to-commit, the coordinate issues a commit event to all subordinates (otherwise an abort event)

• All subordinates commit their works

OCT Research Lab Edward C. Cheng

CS 415

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Recovery

• Checkpoint – the point where all committed changes are flushed out to data pages

• Undo record: enough information to rollback the change (insert, delete, update) – mainly before and after images

• Redo record: enough information to re-execute un-flushed changes

• Upon commit, only make sure to flush out the log records so that upon recovery (after a crash), the system can redo the changes

• Changes might be flushed out to disk before commit. Locks are used to protect us from dirty read. Undo records help to undo uncommitted transactions upon recovery

• Question: what if you have changes that cannot be undone?

OCT Research Lab Edward C. Cheng

CS 415

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