پاورپوینت کامل Chapter 17: Recovery System 79 اسلاید در PowerPoint

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پاورپوینت کامل Chapter 17: Recovery System 79 اسلاید در PowerPoint

اسلاید ۴: Recovery AlgorithmsRecovery algorithms are techniques to ensure database consistency and transaction atomicity and durability despite failuresFocus of this chapterRecovery algorithms have two partsActions taken during normal transaction processing to ensure enough information exists to recover from failuresActions taken after a failure to recover the database contents to a state that ensures atomicity, consistency and durability

اسلاید ۵: Storage StructureVolatile storage:does not survive system crashesexamples: main memory, cache memoryNonvolatile storage:survives system crashesexamples: disk, tape, flash memory, non-volatile (battery backed up) RAM Stable storage:a mythical form of storage that survives all failuresapproximated by maintaining multiple copies on distinct nonvolatile media

اسلاید ۶: Stable-Storage ImplementationMaintain multiple copies of each block on separate diskscopies 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 inSuccessful completionPartial failure: destination block has incorrect informationTotal failure: destination block was never updatedProtecting storage media from failure during data transfer (one solution):Execute output operation as follows (assuming two copies of each block):Write the information onto the first physical block.When the first write successfully completes, write the same information onto the second physical block.The output is completed only after the second write successfully completes.

اسلاید ۷: 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:First find inconsistent blocks:Expensive solution: Compare the two copies of every disk block.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 systemsIf 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.

اسلاید ۸: Data AccessPhysical 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. Tis 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.

اسلاید ۹: 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.

اسلاید ۱۰: Example of Data AccessxYABx1y1 bufferBuffer Block A Buffer Block Binput(A)output(B) read(X)write(Y)diskwork areaof T1work areaof T2 memoryx2

اسلاید ۱۱: Recovery and AtomicityModifying 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.

اسلاید ۱۲: Recovery and Atomicity (Cont.)To ensure atomicity despite failures, we first output information describing the modifications to stable storage without modifying the database itself.We study two approaches:log-based recovery, andshadow-pagingWe assume (initially) that transactions run serially, that is, one after the other.

اسلاید ۱۳: Log-Based RecoveryA log is kept on stable storage. The log is a sequence of log records, and maintains a record of update activities on the database.When transaction Ti starts, it registers itself by writing a <Ti start>log recordBefore Ti executes write(X), a log record <Ti, X, V1, V2> is written, where V1 is the value of X before the write, and V2 is the value to be written to X.Log record notes that Ti has performed a write on data item Xj Xj had value V1 before the write, and will have value V2 after the write. When Ti finishes it last statement, the log record <Ti commit> is written. We assume for now that log records are written directly to stable storage (that is, they are not buffered)Two approaches using logsDeferred database modificationImmediate database modification

اسلاید ۱۴: Deferred Database ModificationThe deferred database modification scheme records all modifications to the log, but defers all the writes to after partial commit.Assume that transactions execute seriallyTransaction starts by writing <Ti start> record to log. A write(X) operation results in a log record <Ti, X, V> being written, where V is the new value for XNote: old value is not needed for this schemeThe write is not performed on X at this time, but is deferred.When Ti partially commits, <Ti commit> is written to the log Finally, the log records are read and used to actually execute the previously deferred writes.

اسلاید ۱۵: Deferred Database Modification (Cont.)During recovery after a crash, a transaction needs to be redone if and only if both <Ti start> and<Ti commit> are there in the log.Redoing a transaction Ti ( redoTi) sets the value of all data items updated by the transaction to the new values.Crashes can occur while the transaction is executing the original updates, or while recovery action is being takenexample transactions T0 and T1 (T0 executes before T1):T0: read (A)T1 : read (C)A: – A – 50 C:-C- 100Write (A) write (C)read (B)B:- B + 50write (B)

اسلاید ۱۶: Deferred Database Modification (Cont.)Below we show the log as it appears at three instances of time.If log on stable storage at time of crash is as in case:(a) No redo actions need to be taken(b) redo(T0) must be performed since <T0 commit> is present (c) redo(T0) must be performed followed by redo(T1) since <T0 commit> and <Ti commit> are present

اسلاید ۱۷: Immediate Database ModificationThe immediate database modification scheme allows database updates of an uncommitted transaction to be made as the writes are issuedsince undoing may be needed, update logs must have both old value and new valueUpdate log record must be written before database item is writtenWe assume that the log record is output directly to stable storageCan be extended to postpone log record output, so long as prior to execution of an output(B) operation for a data block B, all log records corresponding to items B must be flushed to stable storageOutput of updated blocks can take place at any time before or after transaction commitOrder in which blocks are output can be different from the order in which they are written.

اسلاید ۱۸: Immediate Database Modification ExampleLog Write Output<T0 start><T0, A, 1000, 950>To, B, 2000, 2050 A = 950 B = 2050<T0 commit><T1 start><T1, C, 700, 600> C = 600 BB, BC<T1 commit> BANote: BX denotes block containing X.x1

اسلاید ۱۹: Immediate Database Modification (Cont.)Recovery procedure has two operations instead of one: undo(Ti) restores the value of all data items updated by Ti to their old values, going backwards from the last log record for Tiredo(Ti) sets the value of all data items updated by Ti to the new values, going forward from the first log record for TiBoth operations must be idempotentThat is, even if the operation is executed multiple times the effect is the same as if it is executed onceNeeded since operations may get re-executed during recovery When recovering after failure:Transaction Ti needs to be undone if the log contains the record <Ti start>, but does not contain the record <Ti commit>.Transaction Ti needs to be redone if the log contains both the record <Ti start> and the record <Ti commit>.Undo operations are performed first, then redo operations.

اسلاید ۲۰: Immediate DB Modification Recovery Example Below we show the log as it appears at three instances of time.Recovery actions in each case above are:(a) undo (T0): B is restored to 2000 and A to 1000.(b) undo (T1) and redo (T0): C is restored to 700, and then A and B are set to 950 and 2050 respectively.(c) redo (T0) and redo (T1): A and B are set to 950 and 2050 respectively. Then C is set to 600

اسلاید ۲۱: CheckpointsProblems in recovery procedure as discussed earlier :searching the entire log is time-consumingwe might unnecessarily redo transactions which have alreadyoutput their updates to the database.Streamline recovery procedure by periodically performing checkpointing Output all log records currently residing in main memory onto stable storage.Output all modified buffer blocks to the disk.Write a log record < checkpoint> onto stable storage.

اسلاید ۲۲: Checkpoints (Cont.)During recovery we need to consider only the most recent transaction Ti that started before the checkpoint, and transactions that started after Ti. Scan backwards from end of log to find the most recent <checkpoint> record Continue scanning backwards till a record <Ti start> is found. Need only consider the part of log following above start record. Earlier part of log can be ignored during recovery, and can be erased whenever desired.For all transactions (starting from Ti or later) with no <Ti commit>, execute undo(Ti). (Done only in case of immediate modification.)Scanning forward in the log, for all transactions starting from Ti or later with a <Ti commit>, execute redo(Ti).

اسلاید ۲۳: Example of CheckpointsT1 can be ignored (updates already output to disk due to checkpoint)T2 and T3 redone.T4 undoneTcTfT1T2T3T4checkpointsystem failure

اسلاید ۲۴: Shadow PagingShadow paging is an alternative to log-based recovery; this scheme is useful if transactions execute seriallyIdea: maintain two page tables during the lifetime of a transaction –the current page table, and the shadow page tableStore the shadow page table in nonvolatile storage, such that state of the database prior to transaction execution may be recovered. Shadow page table is never modified during executionTo start with, both the page tables are identical. Only current page table is used for data item accesses during execution of the transaction.Whenever any page is about to be written for the first timeA copy of this page is made onto an unused page. The current page table is then made to point to the copyThe update is performed on the copy

اسلاید ۲۵: Sample Page Table

اسلاید ۲۶: Example of Shadow PagingShadow and current page tables after write to page 4

اسلاید ۲۷: Shadow Paging (Cont.)To commit a transaction : 1. Flush all modified pages in main memory to disk 2. Output current page table to disk 3. Make the current page table the new shadow page table, as follows:keep a pointer to the