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