Chapter 16:Recovery System Failure Classification Storage Structure Recovery and Atomicity Log-Based Recovery Remote Backup Systems Database System Concepts-6th Edition 16.2 @Silberschatz,Korth and Sudarshan
Database System Concepts - 6 16.2 ©Silberschatz, Korth and Sudarshan th Edition Chapter 16: Recovery System Failure Classification Storage Structure Recovery and Atomicity Log-Based Recovery 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-6th Edition 16.3 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 16.3 ©Silberschatz, Korth and Sudarshan th Edition 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 Consider transaction 7;that transfers $50 from account A to account B Two updates:subtract 50 from A and add 50 to B Transaction T;requires updates to a and B to be output to the database. A failure may occur after one of these modifications have been made but before both of them are made. Modifying the database without ensuring that the transaction will commit may leave the database in an inconsistent state Not modifying the database may result in lost updates if failure occurs just after transaction commits 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-6th Edition 16.4 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 16.4 ©Silberschatz, Korth and Sudarshan th Edition Recovery Algorithms Consider transaction Ti that transfers $50 from account A to account B Two updates: subtract 50 from A and add 50 to B Transaction Ti requires updates to A and B to be output to the database. A failure may occur after one of these modifications have been made but before both of them are made. Modifying the database without ensuring that the transaction will commit may leave the database in an inconsistent state Not modifying the database may result in lost updates if failure occurs just after transaction commits 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 but may still fail,losing data Stable storage: a mythical form of storage that survives all failures approximated by maintaining multiple copies on distinct nonvolatile media See book for more details on how to implement stable storage Database System Concepts-6th Edition 16.5 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 16.5 ©Silberschatz, Korth and Sudarshan th Edition 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 but may still fail, losing data Stable storage: a mythical form of storage that survives all failures approximated by maintaining multiple copies on distinct nonvolatile media See book for more details on how to implement stable storage
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-6th Edition 16.6 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 16.6 ©Silberschatz, Korth and Sudarshan th Edition 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-6th Edition 16.7 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 16.7 ©Silberschatz, Korth and Sudarshan th Edition 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. We assume,for simplicity,that each data item fits in,and is stored inside,a single block. Database System Concepts-6th Edition 16.8 @Silberschatz,Korth and Sudarshan
Database System Concepts - 6 16.8 ©Silberschatz, Korth and Sudarshan th Edition 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. We assume, for simplicity, that each data item fits in, and is stored inside, a single block
Example of Data Access buffer Buffer Block A input(A) A Buffer Block B B output(B) read(X) write(Y) 2☐ work area work area of T1 of T2 memory disk Database System Concepts-6th Edition 16.9 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 16.9 ©Silberschatz, Korth and Sudarshan th Edition 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
Data Access(Cont.) Each transaction T;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. Transferring data items between system buffer blocks and its private work-area done by: read(X)assigns the value of data item X to the local variable xj. write(X)assigns the value of local variable xi to data item {X)in the buffer block. Note:output(B)need not immediately follow write(X).System can perform the output operation when it deems fit. Transactions Must perform read(X)before accessing X for the first time (subsequent reads can be from local copy) write(X)can be executed at any time before the transaction commits Database System Concepts-6th Edition 16.10 @Silberschatz,Korth and Sudarshan
Database System Concepts - 6 16.10 ©Silberschatz, Korth and Sudarshan th Edition Data Access (Cont.) 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 . Transferring data items between system buffer blocks and its private work-area done by: 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. Note: output(BX) need not immediately follow write(X). System can perform the output operation when it deems fit. Transactions Must perform read(X) before accessing X for the first time (subsequent reads can be from local copy) write(X) can be executed at any time before the transaction commits
Recovery and Atomicity To ensure atomicity despite failures,we first output information describing the modifications to stable storage without modifying the database itself. We study log-based recovery mechanisms in detail We first present key concepts And then present the actual recovery algorithm Less used alternative:shadow-copy and shadow-paging(brief details in book) db-pointer db-pointer old copy of shadow-copy old copy of database new copy of database (to be deleted) database (a)Before update (b)After update Database System Concepts-6th Edition 16.11 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 16.11 ©Silberschatz, Korth and Sudarshan th Edition Recovery and Atomicity To ensure atomicity despite failures, we first output information describing the modifications to stable storage without modifying the database itself. We study log-based recovery mechanisms in detail We first present key concepts And then present the actual recovery algorithm Less used alternative: shadow-copy and shadow-paging (brief details in book) shadow-copy