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WEEK FOUR
TOPIC: CRASH RECOVERY
CONTENT:
- Introduction to Aries (analysis, redo and undo)
- Other recovery related data structure
Sub-topic 1
Crash Recovery: Is the process by which the database is moved back to a consistent and usable state. This is done by rolling back incomplete transactions and completing committed transactions that were still in memory when the crash occurred.
To be able to do this, the DBMS maintains a record, called the Log, of all write to the database.
What is Log?
This is the history of actions executed by the DBMS. Physically, Log is a file of records stored in stable storage, which is assumed to survive crashes.
For recovery purposes, every page in the database contains the LSN of the most recent log record that describe a change to this page. This LSN is called the pageLSN. Every log record is given a unique ID called the Log Sequence Number (LSN).
Introduction to Aries (analysis, redo and undo)
ARIES (Algorithm for Recovery and Isolation Exploiting Semantics) is a recovery algorithm that is designed to work with a steal, no-force approach.
When the recovery manager is invoked after a crash, restart proceeds in three phases:
- Analysis: Identifies dirty pages in the buffer pool (i.e. changes that have not been written to disk) and active transactions at the time of the crash.
- Redo: Repeats all actions, starting from an appropriate point in the log, and restores the database state to what it was at the time of the crash.
- Undo: Undoes the actions of transactions that did not commit, so that the database reflects only the actions of committed transactions.
There three main principles behind the ARIES recovery algorithm:
- Write-ahead logging: Any change to a database object is first recorded in the log; the record in the log must be written to stable storage before the change to the database object is written to disk.
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