Chapter 17:Recovery System Failure Classification Storage Structure Recovery and Atomicity Log-Based Recovery Shadow Paging Recovery With Concurrent Transactions Buffer Management Failure with Loss of Nonvolatile Storage Advanced Recovery Techniques ARIES Recovery Algorithm Remote Backup Systems Database System Concepts,5th Edition,Oct 5,2006 17.2 @Silberschatz,Korth and Sudarshan
Database System Concepts, 5 17.2 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Chapter 17: Recovery System Failure Classification Storage Structure Recovery and Atomicity Log-Based Recovery Shadow Paging Recovery With Concurrent Transactions Buffer Management Failure with Loss of Nonvolatile Storage Advanced Recovery Techniques ARIES Recovery Algorithm Remote Backup Systems
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 Database System Concepts,5th Edition,Oct 5,2006 17.3 ©Silberschat乜，Korth and Sudarshan
Database System Concepts, 5 17.3 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 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
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 Database System Concepts,5th Edition,Oct 5,2006 17.4 ©Silberschat乜，Korth and Sudarshan
Database System Concepts, 5 17.4 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 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
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 Database System Concepts,5th Edition,Oct 5,2006 17.5 @Silberschatz,Korth and Sudarshan
Database System Concepts, 5 17.5 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 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
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. 2.When the first write successfully completes,write the same information onto the second physical block. 3. The output is completed only after the second write successfully completes. Database System Concepts,5th Edition,Oct 5,2006 17.6 ©Silberschat乜，Korth and Sudarshan
Database System Concepts, 5 17.6 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 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. 2. When the first write successfully completes, write the same information onto the second physical block. 3. The output is completed only after the second write successfully completes
Stable-Storage Implementation(Cont.) Protecting storage media from failure during data transfer(cont.) Copies of a block may differ due to failure during output operation.To recover from failure: 1.First find inconsistent blocks: 1. Expensive solution:Compare the two copies of every disk block. 2.Better solution: Record in-progress disk writes on non-volatile storage(Non- volatile RAM or special area of disk). Use this information during recovery to find blocks that may be inconsistent,and only compare copies of these. Used in hardware RAID systems 2.If either copy of an inconsistent block is detected to have an error(bad checksum),overwrite it by the other copy.If both have no error,but are different,overwrite the second block by the first block. Database System Concepts,5th Edition,Oct 5,2006 17.7 @Silberschatz,Korth and Sudarshan
Database System Concepts, 5 17.7 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Stable-Storage Implementation (Cont.) Protecting storage media from failure during data transfer (cont.): Copies of a block may differ due to failure during output operation. To recover from failure: 1. First find inconsistent blocks: 1. Expensive solution: Compare the two copies of every disk block. 2. Better solution: Record in-progress disk writes on non-volatile storage (Nonvolatile RAM or special area of disk). Use this information during recovery to find blocks that may be inconsistent, and only compare copies of these. Used in hardware RAID systems 2. If either copy of an inconsistent block is detected to have an error (bad checksum), overwrite it by the other copy. If both have no error, but are different, overwrite the second block by the first block
Data Access Physical blocks are those blocks residing on the disk. Buffer blocks are the blocks residing temporarily in main memory. Block movements between disk and main memory are initiated through the following two operations: input(B)transfers the physical block B to main memory. output(B)transfers the buffer block B to the disk,and replaces the appropriate physical block there. Each transaction Ti has its private work-area in which local copies of all data items accessed and updated by it are kept. Ti's local copy of a data item X is called xi. We assume,for simplicity,that each data item fits in,and is stored inside,a single block. Database System Concepts,5th Edition,Oct 5,2006 17.8 @Silberschatz,Korth and Sudarshan
Database System Concepts, 5 17.8 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Data Access Physical blocks are those blocks residing on the disk. Buffer blocks are the blocks residing temporarily in main memory. Block movements between disk and main memory are initiated through the following two operations: input(B) transfers the physical block B to main memory. output(B) transfers the buffer block B to the disk, and replaces the appropriate physical block there. Each transaction Ti has its private work-area in which local copies of all data items accessed and updated by it are kept. Ti 's local copy of a data item X is called xi . We assume, for simplicity, that each data item fits in, and is stored inside, a single block
Data Access(Cont.) Transaction transfers data items between system buffer blocks and its private work-area using the following operations read(X)assigns the value of data item X to the local variable Xj. write(X)assigns the value of local variable x;to data item {X)in the buffer block. both these commands may necessitate the issue of an input(Bx) instruction before the assignment,if the block By in which X resides is not already in memory. Transactions Perform read(X)while accessing X for the first time; All subsequent accesses are to the local copy. After last access,transaction executes write(X). output(Bx)need not immediately follow write(X).System can perform the output operation when it deems fit. Database System Concepts,5th Edition,Oct 5,2006 17.9 @Silberschatz,Korth and Sudarshan
Database System Concepts, 5 17.9 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Data Access (Cont.) Transaction transfers data items between system buffer blocks and its private work-area using the following operations : read(X) assigns the value of data item X to the local variable xi . write(X) assigns the value of local variable xi to data item {X} in the buffer block. both these commands may necessitate the issue of an input(BX) instruction before the assignment, if the block BX in which X resides is not already in memory. Transactions Perform read(X) while accessing X for the first time; All subsequent accesses are to the local copy. After last access, transaction executes write(X). output(BX) need not immediately follow write(X). System can perform the output operation when it deems fit
Example of Data Access buffer Buffer Block A input(A) A Buffer Block B Y B output(B) read(X) write(Y)） x□ 2□ y work area work area of T1 of T2 memory disk Database System Concepts,5th Edition,Oct 5,2006 17.10 ©Silberschat乜，Korth and Sudarshan
Database System Concepts, 5 17.10 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Example of Data Access X Y A B x1 y1 buffer Buffer Block A Buffer Block B input(A) output(B) read(X) write(Y) disk work area of T1 work area of T2 memory x2
Recovery and Atomicity Modifying the database without ensuring that the transaction will commit may leave the database in an inconsistent state. Consider transaction T;that transfers $50 from account A to account B; goal is either to perform all database modifications made by 7;or none at all. Several output operations may be required for T;(to output A and B).A failure may occur after one of these modifications have been made but before all of them are made. Database System Concepts,5th Edition,Oct 5,2006 17.11 ©Silberschat乜，Korth and Sudarshan
Database System Concepts, 5 17.11 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Recovery and Atomicity Modifying the database without ensuring that the transaction will commit may leave the database in an inconsistent state. Consider transaction Ti that transfers $50 from account A to account B; goal is either to perform all database modifications made by Ti or none at all. Several output operations may be required for Ti (to output A and B). A failure may occur after one of these modifications have been made but before all of them are made