Database Recovery Introduction

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Questions and Answers

What is the first step in recovering from a failure in a database system?

Record all disk writes on volatile storage.

Find inconsistent blocks. (correct)

Overwrite damaged blocks with backup copies.

Transfer data items to main memory.

Which of the following best describes the purpose of using non-volatile storage during recovery?

To create backups of all database transactions.

To speed up disk writes.

To store temporary copies of data.

To record in-progress disk writes. (correct)

What must be done if both copies of an inconsistent block are error-free but differ?

Keep the copy with the latest timestamp.

Create a new copy of both blocks.

Overwrite the second block with the first block. (correct)

Ignore the discrepancies.

What operation transfers a physical block to main memory?

input(B) Signup and view all the answers

What is a local copy of a data item accessed by transaction Ti called?

xi Signup and view all the answers

Which operation assigns a value from a data item to a local variable in a transaction?

read(X) Signup and view all the answers

What occurs during the output operation in data access?

Buffer block B is replaced on the disk. Signup and view all the answers

What characterizes physical blocks in a database system?

They reside on the disk. Signup and view all the answers

What must occur before another transaction can modify an item that has been updated by transaction Ti?

Transaction Ti must commit or abort. Signup and view all the answers

Which operation restores the value of all data items updated by a transaction to their old values?

undo(Ti) Signup and view all the answers

What is the primary purpose of logging during undo operations?

To track changes for potential rollbacks Signup and view all the answers

What mechanism ensures that the updates of uncommitted transactions remain invisible to others?

Strict two-phase locking Signup and view all the answers

When performing a redo operation, what happens with logging?

No logging is done. Signup and view all the answers

Under what circumstance must transaction Ti be undone during recovery?

If the log contains the record but lacks the commit record. Signup and view all the answers

What is the effect of obtaining exclusive locks on items during transactions?

It prevents other transactions from modifying the same item until the transaction ends. Signup and view all the answers

What is written out when the undo operation for a transaction is complete?

An undo log record Signup and view all the answers

What type of logging allows early lock release on space allocation information?

Logical undo logging Signup and view all the answers

Which statement accurately describes physical redo logging?

It logs the new value for each write. Signup and view all the answers

What happens when an operation is initiated in operation logging?

A unique identifier for the operation is logged. Signup and view all the answers

What does logical undo logging require when logging an operation?

Both physical redo and physical undo information. Signup and view all the answers

Which example illustrates the concept of logical operations in undo logging?

Subtracting an amount deposited to undo the deposit. Signup and view all the answers

What is a potential complication of logical redo logging?

Database states may not be operation consistent. Signup and view all the answers

In operation logging, what is logged upon the completion of an operation?

The logical undo information. Signup and view all the answers

Which statement is NOT true regarding physical redo logging?

It guarantees logical consistency after recovery. Signup and view all the answers

What is the purpose of the UndoNextLSN value in a compensation log record (CLR)?

To indicate the next log record to undo Signup and view all the answers

During an undo pass, how is the next log sequence number (LSN) chosen for undoing a transaction?

By picking the largest of the last log records for each transaction Signup and view all the answers

What does ARIES support regarding transaction rollback?

Both partial and full rollback of transactions Signup and view all the answers

How does the undo pass optimize the backward scan on the log?

By skipping unneeded log records during the scan Signup and view all the answers

What triggers a partial rollback in ARIES?

Deadlocks requiring the release of locks Signup and view all the answers

What is the benefit of recovery independence in ARIES?

Pages can be recovered independently of each other Signup and view all the answers

What happens to the next LSN after undoing an ordinary log record?

It is updated to the PrevLSN noted in the log record Signup and view all the answers

In the context of ARIES, what is a compensation log record (CLR) primarily used for?

To record the steps needed for future undo operations Signup and view all the answers

What is a significant drawback of implementing the database buffer in reserved main-memory?

Memory is partitioned before-hand, limiting flexibility. Signup and view all the answers

What is known as the dual paging problem?

When the operating system evicts a modified page to swap space. Signup and view all the answers

In which memory does the database buffer generally get implemented despite certain drawbacks?

Virtual memory Signup and view all the answers

What is the first step in fuzzy checkpointing?

Temporarily stop all updates by transactions. Signup and view all the answers

What should ideally happen when the operating system needs to evict a page from the buffer?

The database should output the page to disk instead of swap space. Signup and view all the answers

What happens when the operating system needs to evict a modified page that is currently in swap space?

It must be read from swap space to write to the database, increasing I/O. Signup and view all the answers

What is the purpose of implementing a database buffer in virtual memory?

To allow for dynamic memory allocation as needed. Signup and view all the answers

Why is it ideal for the database to output modified pages directly to disk?

To avoid going through swap space, which adds unnecessary overhead. Signup and view all the answers

What is the primary advantage of shadow paging over log-based recovery?

It allows for immediate recovery without the need for logs. Signup and view all the answers

During a transaction, what is the role of the shadow page table?

It is identical to the current page table at the beginning of the transaction. Signup and view all the answers

What happens to the current page table when a page is about to be written for the first time?

A copy of the page is created and the current table updates its pointer. Signup and view all the answers

Which of the following steps is NOT part of committing a transaction in shadow paging?

Deleting all pages in the current page table. Signup and view all the answers

After a crash, what can new transactions use for proceeding in shadow paging?

The shadow page table that was last saved. Signup and view all the answers

What occurs if pages are not pointed to by the current or shadow page table?

They should be subject to garbage collection. Signup and view all the answers

How are modified pages managed during transaction execution in shadow paging?

Each modified page is copied to an unused page. Signup and view all the answers

What is the significance of the pointer to the shadow page table?

It enables quick access to the last valid state of the database. Signup and view all the answers

Study Notes

Chapter 19: Recovery System

This chapter discusses recovery systems in database systems.
Failure classifications are discussed
- Transaction failures:
  - Logical errors: A transaction cannot complete due to an internal error condition.
  - System errors: The database system terminates an active transaction due to an error condition (e.g., deadlock).
- System crashes: Power failure or other hardware/software failures causing the system to crash,
- Disk failure: Head crash or similar disk failure destroying disk storage.
Recovery algorithms have two primary parts:
- Actions taken during normal transaction processing to ensure sufficient information for recovery from future failures
- Actions taken after a failure to restore the database to a state ensuring atomicity, consistency, and durability
Storage Structure is detailed:
- Volatile storage: Does not survive system crashes (e.g., main memory, cache memory).
- Nonvolatile storage: Survives system crashes (e.g., disk, tape, flash memory, nonvolatile RAM) but can still fail and lose data.
- Stable storage: A theoretical storage that survives all failures. approximated using multiple, nonvolatile copies.
Stable-Storage Implementation techniques:
- Maintain multiple copies of each block on separate disks to protect against disaster (e.g., fire or flooding).
- Execute operations (writing to both copies of each block) allowing for detection and recovery from data transfer failures.
Recovery Algorithms are discussed:
- Suppose a transaction transfers money between accounts, and one update fails. The recovery algorithm needs to correctly account for partial transactions.
- Recovery algorithms have 2 key components
  - Actions to perform during normal operations to create enough information to recover after a failure
  - Actions to execute after a failure restore data to an atomic, consistent, and durable state
Data Access explains physical and buffer blocks, along with input/output operations between main memory and disk.
Each transaction has its work area keeping local copies of accessed data for recovery. Data transfer involves using read and write operations.
Recovery and Atomicity: ensures atomicity despite failures by first outputting modification information to stable storage without changing the database directly, and studying log-based recovery mechanisms.
Log-Based Recovery describes logs as sequences of log records tracking modifications for atomicity.
- When a transaction starts, a <T; start> log record is written and registers the transaction.
- Before a write(X) operation, a log record <T₁, X, V1, V2> is written to the log (V1 is the old value of X, V2 is the new value). When a transaction finishes execution a <T₁ commit> or <T₁ abort> log record is written.
- Two approaches to log use: Immediate and deferred database modifications.
Immediate Database Modification:
- Updates occur to buffer or disk right away before the transaction commits.
- Output of updated blocks may occur in a different order than the order the updates were written, to disk
Deferred Database Modification:
- Performs updates to buffer or disk only at the time that a transaction commits
- Less overhead, but may have more storage overhead for local copies.
Transaction Commit requires that all prior log records need to have been output to stable storage before the commit log record.
Concurrency Control and Recovery: Concurrent transactions share the same disk buffer and log, and data updates from uncommitted transactions will be invisible. Operations are performed with locks to avoid issues.
Undo and Redo Operations describe how to restore or repeat a transaction's updates either going backwards (undo) or forwards (redo) based on available log records or data.
Recovering from Failure explains recovery actions when a failure occurs, and if a transaction needed to be undone or redone. Recovery involves restoring the database to a consistent or previous state if earlier actions were undone due to failure,
Checkpoints describe how periodic logging procedures can reduce recovery overhead
- Periodically all updated records in memory and buffer get logged to stable storage, allowing later recovery procedures to start recovery from a checkpoint point, rather than from the beginning of the log.
Fuzzy Checkpointing: Temporary interruption in normal processing activities for updates while performing check-pointing to allow updates to take place during the procedure.
Failure with Loss of Nonvolatile Storage discusses how to deal with failures in non-volatile storage, such as a disk failure. Recovery in this case involves restoring the database from a prior dump; checking the log to identify completed transactions, and repeating actions in this case.
Remote Backup Systems describe systems offering high availability where transaction processing can continue even if the primary site is destroyed.
- Detection of failure: Backup sites detect primary site failure through communication links (e.g., heartbeat messages) and distinguish links from failures to prevent incorrect recovery.
- Transfer control is handled by first completing recovery on a backup copy of the database and received log records.
- Recovery time is reduced by periodic backups and periodic log processes.
Recovery with Early Lock Release and Logical Undo Operations:
- High-concurrency locking techniques and release locks early, leading to logical undo in this recovery scheme. Operations are redone as normal.
Logical Undo Logging: A database recovery method that executes undoing operations (deletion), rather than restoring to previous values (physical).
Physical Redo: Redo information is logged physically (new value for each write) to avoid the complexities of logical redo.
Operation Logging method for logging operations (starts and ends):
- A unique operation identifier (Oj) is used for logging.
- Records with physical redo and undo information are logged during operation execution.
- Includes operation-begin and operation-end markers.
Transaction Rollback with Logical Undo: Describes how transaction rollback is done during normal operations.
Recovery Algorithm with Logical Undo involves redo and undo phases after a crash
ARIES Recovery Algorithm: A recovery method that uses optimization techniques, including log sequence numbers, to speed up the recovery process. It includes three passes (Analysis, Redo and Undo).
ARIES Data Structures: Includes log sequence numbers (LSN), dirty page tables, and checkpoint log, to increase efficiency.
Checkpoint Log tracks what changes have already been performed physically.
ARIES Recovery Algorithm, also involves three passes to determine which transactions need to be rolled back or redone.
- Analysis phase: Identifies transactions that need undo or redo, identifies the LSN for the redo to start from.
- Redo phase: Re-executes actions from the redoLSN, avoiding re-doing actions already done.
- Undo phase: Re-executes undo actions for incomplete transactions
Other ARIES Features may involve fine-grained locking, recovery independence, and savepoints, which allow for more sophisticated concurrency control and recovery.
Recovery in Main Memory Databases are recovery processes on main memory databases and optimizations.
Shadow Paging; a system to copy pages and recover from changes; efficient for serial execution but it is expensive for copying a large table.
Block Storage explains how data is stored and retrieved on disk and in memory, using input (A) and output (B) operations for managing transfer.

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Description

This quiz explores essential concepts related to database recovery processes, focusing on non-volatile storage, transaction management, and data item access. Learn about the operations involved in restoring, undoing, and logging changes in a database system. Prepare to understand critical recovery mechanisms and their significance.