Recovery - Database System Concepts ©Silberschatz Korth and Sudarshan See www.db-book.com for conditions on re-use Chapter 17 Recovery System

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Unformatted text preview: Database System Concepts ©Silberschatz, Korth and Sudarshan See www.db-book.com for conditions on re-use Chapter 17: Recovery System Chapter 17: Recovery System ©Silberschatz, Korth and Sudarshan 17.2 Database System Concepts, 5 th Ed. Failure Classification Failure Classification ■ Transaction failure : ● Logical errors : transaction cannot complete due to some internal error condition ● System errors : the database system must terminate an active transaction due to an error condition (e.g., deadlock) ■ System crash : a power failure or other hardware or software failure causes the system to crash. ● Fail-stop assumption : non-volatile storage contents are assumed to not be corrupted by system crash Database systems have numerous integrity checks to prevent corruption of disk data ■ Disk failure : a head crash or similar disk failure destroys all or part of disk storage ● Destruction is assumed to be detectable: disk drives use checksums to detect failures ©Silberschatz, Korth and Sudarshan 17.3 Database System Concepts, 5 th Ed. Recovery Algorithms Recovery Algorithms ■ Recovery algorithms are techniques to ensure database consistency and transaction atomicity and durability despite failures ● Focus of this chapter ■ Recovery algorithms have two parts 1. Actions taken during normal transaction processing to ensure enough information exists to recover from failures 2. Actions taken after a failure to recover the database contents to a state that ensures atomicity, consistency and durability ©Silberschatz, Korth and Sudarshan 17.4 Database System Concepts, 5 th Ed. Storage Structure Storage Structure ■ Volatile storage : ● does not survive system crashes ● examples: main memory, cache memory ■ Nonvolatile storage : ● survives system crashes ● examples: disk, tape, flash memory, non-volatile (battery backed up) RAM ■ Stable storage : ● a mythical form of storage that survives all failures ● approximated by maintaining multiple copies on distinct nonvolatile media ©Silberschatz, Korth and Sudarshan 17.5 Database System Concepts, 5 th Ed. Stable-Storage Implementation Stable-Storage Implementation ■ Maintain multiple copies of each block on separate disks ● copies can be at remote sites to protect against disasters such as fire or flooding. ■ Failure during data transfer can still result in inconsistent copies: Block transfer can result in ● Successful completion ● Partial failure: destination block has incorrect information ● Total failure: destination block was never updated ■ Protecting storage media from failure during data transfer (one solution): ● Execute output operation as follows (assuming two copies of each block): 1. Write the information onto the first physical block....
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This note was uploaded on 02/17/2010 for the course ACS ACS-3911 taught by Professor Chen during the Spring '10 term at Windsor.

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Recovery - Database System Concepts ©Silberschatz Korth and Sudarshan See www.db-book.com for conditions on re-use Chapter 17 Recovery System

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