Recovery System Dbms

17. convalescence System in entropybase management system Presentation Transcript 1. Chapter 17 recuperation System * mishap Classification * Storage Structure * Reco truly and Atomicity * pound-Based convalescence * dwarf Paging * convalescence With Concurrent exertions * Buffer precaution * Failure with divergence of Nonvolatile Storage * Advanced Recovery Techniques * read/write memory Recovery Algorithm * conflicting Backup Systems 2. Failure Classification * Transaction failure * Logical misappre hensions doing post non complete due to some internal error limit * System errors the infobase system moldiness terminate an vigorous exercise due to an error condition (e. . , deadlock) * System scoot a power failure or some other hardw ar or softw atomic number 18 failure ca affairs the system to crash. * Fail-stop assumption non-volatile shop contents be assumed to not be misdirect by system crash * infobase systems confine numerous integrity check s to impede corruption of discus info * Disk failure a head crash or similar plough failure destroys tout ensemble or eccentric of magnetic disk computer memory * Destruction is assumed to be detec delay disk drives use checksums to detect failures 3. Recovery Algorithms Recovery algorithms are proficiencys to en for sure entropybase consistency and dealings atomicity and durability despite failures * counselling of this chapter * Recovery algorithms exact two parts * Actions taken during normal consummation touch on to ensure liberal information exists to bump from failures * Actions taken aft(prenominal) a failure to recover the database contents to a soil that ensures atomicity, consistency and durability 4. Storage Structure * Volatile retentiveness * does not survive system crashes * illustrations principal(prenominal) remembrance, cache memory * Nonvolatile storage survives system crashes * examples disk, tape, flash memory, non-volatile (battery back ed up) RAM * S panel storage * a mythical form of storage that survives completely failures * approximated by maintaining multiple copies on distinct nonvolatilisable media 5. S elude-Storage Implementation * Maintain multiple copies of individually tote on separate disks * copies stool be at remote sites to protect against disasters much(prenominal) as fire or flooding. * Failure during data transfer skunk still result in inapposite copies Block transfer can result in * make completion Partial failure oddity read engine interrupt has incorrect information * Total failure destination block was neer updated * Protecting storage media from failure during data transfer (one solution) * Execute produce subprogram as follows (assuming two copies of each block) * Write the information onto the original physical block. * When the first write success wide of the marky completes, write the equal information onto the reciprocal ohm physical block. * The make is holy only subsequently the second write successfully completes. 6.Stable-Storage Implementation (Cont. ) * Protecting storage media from failure during data transfer (cont. ) * Copies of a block may differ due to failure during getup subroutine. To recover from failure * First bring in consonant blocks * Expensive solution Compare the two copies of either disk block. * Better solution * Record in-progress disk writes on non-volatile storage (Non-volatile RAM or surplus area of disk). * Use this information during recuperation to find blocks that may be inconsistent, and only compare copies of these. Used in hardware RAID systems * If either model of an inconsistent block is notice to assimilate an error (bad checksum), overwrite it by the other retroflex. If both have no error, still are different, overwrite the second block by the first block. 7. entropy admission charge * Physical blocks are those blocks residing on the disk. * Buffer blocks are the blocks residing tempor arily in main memory. * Block movements in the midst of disk and main memory are initiated through the pastime two trading exertions * input ( B ) transfers the physical block B to main memory. output ( B ) transfers the fender block B to the disk, and replaces the discriminate physical block there. * Each transaction T i has its private work-area in which local copies of whole data specifics accessed and updated by it are kept. * T i s local copy of a data item X is c wholeed x i . * We assume, for simplicity, that each data item fits in, and is stored inside, a single block. 8. Data Access (Cont. ) * Transaction transfers data items between system pilot program blocks and its private work-area employ the pursuance operations * read ( X ) assigns the rate of data item X to the local variable x i . write ( X ) assigns the value of local variable x i to data item X in the pilot burner block. * both these commands may necessitate the issue of an input (B X ) instructi on in front the assignment, if the block B X in which X resides is not already in memory. * rifleings * practise read ( X ) musical composition accessing X for the first time * every subsequent accesses are to the local copy. * After come through access, transaction executes write ( X ). * output ( B X ) need not neighboring(a)ly follow write ( X ).System can perform the output operation when it deems fit. 9. caseful of Data Access x Y A B x 1 y 1 buff store Buffer Block A Buffer Block B input(A) output(B) read(X) write(Y) disk work area of T 1 work area of T 2 memory x 2 10. Recovery and Atomicity * Modifying the database without ensuring that the transaction will commit may leave the database in an inconsistent cite. * Consider transaction T i that transfers $50 from account A to account B goal is either to perform both database modifications made by T i or none at entirely. Several output operations may be postulate for T i (to output A and B ). A failure may occu r ulterior on one of these modifications have been made but origin all(prenominal)y all of them are made. 11. Recovery and Atomicity (Cont. ) * To ensure atomicity despite failures, we first output information describing the modifications to inactive storage without modifying the database itself. * We consume two approaches * put down- ground recovery , and * tail assembly-paging * We assume (initially) that proceedings run serially, that is, one after the other. 12. Log-Based Recovery A pound is kept on constant storage. * The put down is a sequence of enter discharges , and maintains a write down of update activities on the database. * When transaction T i readinessoffs, it registers itself by writing a T i break up enter bring down * Before T i executes write ( X ), a account book study T i , X, V 1 , V 2 is written, where V 1 is the value of X before the write, and V 2 is the value to be written to X . * Log insert notes that T i has performed a write on data item X j X j had value V 1 before the write, and will have value V 2 after the write. When T i finishes it last statement, the pound record T i commi t is written. * We assume for instanter that put down records are written directly to invariable storage (that is, they are not buffered) * Two approaches using enters * Deferred database modification * spry database modification 13. Deferred Database Modification * The deferred database modification scheme records all modifications to the log, but defers all the write s to after partial commit. * Assume that proceedings execute serially Transaction starts by writing T i start record to log. * A write ( X ) operation results in a log record T i , X, V being written, where V is the unsanded value for X * preeminence sexagenarian value is not needed for this scheme * The write is not performed on X at this time, but is deferred. * When T i partially commits, T i commit is written to the log * Finally, the log records are read and utilize to actually execute the previously deferred writes. 14. Deferred Database Modification (Cont. ) During recovery after a crash, a transaction needs to be refashionne if and only if both T i start and T i commit are there in the log. * recasting a transaction T i ( redo T i ) sets the value of all data items updated by the transaction to the new values. * Crashes can occur maculation * the transaction is executing the original updates, or * while recovery action is being taken * example transactions T 0 and T 1 ( T 0 executes before T 1 ) * T 0 read ( A ) T 1 read ( C ) * A A 50 C- C- 100 Write ( A ) write ( C ) * read ( B ) * B- B + 50 * write ( B ) 15. Deferred Database Modification (Cont. ) * Below we show the log as it appears at cardinal 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( T 0 ) moldiness be performed since T 0 commi t is puzzle * (c) redo ( T 0 ) must be performed followed by redo( T 1 ) since * T 0 commit and T i commit are present 16. spry Database Modification The immediate database modification scheme allows database updates of an floating(prenominal) transaction to be made as the writes are issued * since discovering may be needed, update logs must have both old value and new value * Update log record must be written before database item is written * We assume that the log record is output directly to stable storage * Can 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 storage * end product of updated blocks can take place at both time before or after transaction commit * Order in which blocks are output can be different from the clubhouse in which they are written. 17. Immediate Database Modification Example * Log Write Output * T 0 start T 0 , A, 1000, 950 * T o , B, 2000, 2 050 * A = 950 * B = 2050 * T 0 commit * T 1 start * T 1 , C, 700, 600 * C = 600 * B B , B C * T 1 commit * B A * Note B X denotes block containing X . x 1 18. Immediate Database Modification (Cont. ) * Recovery role has two operations instead of one * unwrap ( T i ) restores the value of all data items updated by T i to their old values, going rearwards from the last log record for T i * redo ( T i ) sets the value of all data items updated by T i to the new values, going forward from the first log record for T i * Both operations must be idempotent That is, even if the operation is executed multiple times the effect is the same as if it is executed once * Needed since operations may get re-executed during recovery * When convalescent after failure * Transaction T i needs to be sunk if the log contains the record T i start , but does not contain the record T i commit . * Transaction T i needs to be re do if the log contains both the record T i start and the record T i c ommit . * Undo operations are performed first, consequently redo operations. 19. 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) tease apart ( T 0 ) B is restored to 2000 and A to 1000. (b) undo ( T 1 ) and redo ( T 0 ) C is restored to 700, and consequently A and B are * set to 950 and 2050 respectively. * (c) redo ( T 0 ) and redo ( T 1 ) A and B are set to 950 and 2050 * respectively. Then C is set to 600 20. Checkpoints * Problems in recovery procedure as discussed earlier * searching the entire log is time-consuming * we might unnecessarily redo transactions which have already * output 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 storag e. 1. Checkpoints (Cont. ) * During recovery we need to consider only the more or less recent transaction T i that started before the checkpoint, and transactions that started after T i . * run over indisposeds from end of log to find the most recent checkpoint record * Continue scanning backwards till a record T i start is launch. * Need only consider the part of log following above star t record. Earlier part of log can be ignored during recovery, and can be erased whenever desired. * For all transactions (starting from T i or subsequent) with no T i commit , execute undo ( T i ). (Done only in case of immediate modification. * Scanning forward in the log, for all transactions starting from T i or later with a T i commit , execute redo ( T i ). 22. Example of Checkpoints * T 1 can be ignored (updates already output to disk due to checkpoint) * T 2 and T 3 redone. * T 4 turn T c T f T 1 T 2 T 3 T 4 checkpoint system failure 23. swarthiness Paging * Shadow paging is an a lternative to log-based recovery this scheme is useful if transactions execute serially * Idea maintain two rapscallion tables during the lifetime of a transaction the current paginate table , and the shadow scalawag table * Store the shadow foliate table in nonvolatile storage, such(prenominal) that state of the database prior to transaction execution may be recovered. Shadow rapscallionboy table is never modified during execution * To start with, both the rapscallion 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 time * A copy of this page is made onto an unused page. * The current page table is then made to point to the copy * The update is performed on the copy 24. Sample Page Table 25. Example of Shadow Paging Shadow and current page tables after write to page 4 26. Shadow Paging (Cont. ) * To commit a transaction * 1. Flush all modified pages in m ain 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 shadow page table at a fixed (k straight offn) location on disk. * to make the current page table the new shadow page table, simply update the pointer to point to current page table on disk * Once pointer to shadow page table has been written, transaction is committed. * No recovery is needed after a crash new transactions can start right away, using the shadow page table. * Pages not pointed to from current/shadow page table should be freed (garbage self-collected). 27. Show Paging (Cont. ) * Advantages of shadow-paging over log-based schemes * no overhead of writing log records * recovery is trivial * Disadvantages * Copying the entire page table is very expensive Can be reduced by using a page table unified like a B + -tree * No need to copy entire tree, only need to copy paths in the tree that lead to updated leaf nodes * Com mit overhead is high even with above ack todayledgment * Need to flush every updated page, and page table * Data gets fragmented (related pages get separated on disk) * After every transaction completion, the database pages containing old editions of modified data need to be garbage collected * Hard to extend algorithm to allow transactions to run concurrently * Easier to extend log based schemes 28. Recovery With Concurrent minutes * We modify the log-based recovery schemes to allow multiple transactions to execute concurrently. * whole transactions share a single disk buffer and a single log * A buffer block can have data items updated by one or more transactions * We assume concurrency control using strict two-phase fix * i. e. the updates of uncommitted transactions should not be panoptical to other transactions * Otherwise how to perform undo if T1 updates A, then T2 updates A and commits, and finally T1 has to abort? * log is done as described earlier. Log records of diff erent transactions may be interspersed in the log. * The checkpointing technique and actions taken on recovery have to be changed * since several transactions may be active when a checkpoint is performed. 29. Recovery With Concurrent Transactions (Cont. ) * Checkpoints are performed as before, except that the checkpoint log record is now of the form checkpoint L where L is the add up of transactions active at the time of the checkpoint * We assume no updates are in progress while the checkpoint is carried out (will relax this later) * When the system recovers from a crash, it first does the following * Initialize undo-list and redo-list to empty Scan the log backwards from the end, stopping when the first checkpoint L record is found. For each record found during the backward scan * if the record is T i commit , add T i to redo-list * if the record is T i start , then if T i is not in redo-list , add T i to undo-list * For every T i in L , if T i is not in redo-list , add T i to undo-list 30. Recovery With Concurrent Transactions (Cont. ) * At this point undo-list consists of uncomplete transactions which must be undone, and redo-list consists of finished transactions that must be redone. * Recovery now continues as follows Scan log backwards from most recent record, stopping when T i start records have been encountered for every T i in undo-list . * During the scan, perform undo for each log record that belongs to a transaction in undo-list . * Locate the most recent checkpoint L record. * Scan log forward from the checkpoint L record till the end of the log. * During the scan, perform redo for each log record that belongs to a transaction on redo-list 31. Example of Recovery * Go over the steps of the recovery algorithm on the following log * T 0 star t * T 0 , A , 0, 10 * T 0 commit * T 1 start * T 1 , B , 0, 10 T 2 start /* Scan in Step 4 stops here */ * T 2 , C , 0, 10 * T 2 , C , 10, 20 * checkpoint T 1 , T 2 * T 3 start * T 3 , A , 10, 20 * T 3 , D , 0, 10 * T 3 commit 32. Log Record Buffering * Log record buffering log records are buffered in main memory, instead of of being output directly to stable storage. * Log records are output to stable storage when a block of log records in the buffer is full, or a log force operation is executed. * Log force is performed to commit a transaction by forcing all its log records (including the commit record) to stable storage. Several log records can thus be output using a single output operation, reducing the I/O cost. 33. Log Record Buffering (Cont. ) * The rules below must be followed if log records are buffered * Log records are output to stable storage in the order in which they are created. * Transaction T i enters the commit state only when the log record T i commit has been output to stable storage. * Before a block of data in main memory is output to the database, all log records pertaining to data in that block must have been output to stable st orage. * This rule is called the write-ahead enter or WAL rule * Strictly speaking WAL only requires undo information to be output 34. Database Buffering Database maintains an in-memory buffer of data blocks * When a new block is needed, if buffer is full an existing block needs to be removed from buffer * If the block chosen for removal has been updated, it must be output to disk * As a result of the write-ahead logging rule, if a block with uncommitted updates is output to disk, log records with undo information for the updates are output to the log on stable storage first. * No updates should be in progress on a block when it is output to disk. Can be ensured as follows. * Before writing a data item, transaction acquires exclusive lock on block containing the data item * Lock can be released once the write is completed. * Such locks held for short continuation are called latches . Before a block is output to disk, the system acquires an exclusive latch on the block * Ensures no update can be in progress on the block 35. Buffer Management (Cont. ) * Database buffer can be implemented either * in an area of real main-memory reserved for the database, or * in practical(prenominal) memory * Implementing buffer in reserved main-memory has drawbacks * Memory is partitioned before-hand between database buffer and applications, close flexibility. * Needs may change, and although operate system knows best how memory should be divided up at any time, it cannot change the partitioning of memory. 36. Buffer Management (Cont. ) Database buffers are generally implemented in virtual memory in spite of some drawbacks * When operating(a) system needs to evict a page that has been modified, to make space for another page, the page is written to swap space on disk. * When database decides to write buffer page to disk, buffer page may be in swap space, and may have to be read from swap space on disk and output to the database on disk, resulting in extra I/O * Known as dual paging trouble. * Ideally when swapping out a database buffer page, operating system should pass control to database, which in turn outputs page to database instead of to swap space (making sure to output log records first) * Dual paging can thus be avoided, but common operating systems do not support such functionality. 37. Failure with Loss of Nonvolatile Storage So further we assumed no loss of non-volatile storage * Technique similar to checkpointing used to deal with loss of non-volatile storage * Periodically dump the entire content of the database to stable storage * No transaction may be active during the dump procedure a procedure similar to checkpointing must take place * Output all log records currently residing in main memory onto stable storage. * Output all buffer blocks onto the disk. * Copy the contents of the database to stable storage. * Output a record dump to log on stable storage. * To recover from disk failure * restore database from most recent dump. Cons ult the log and redo all transactions that committed after the dump * Can be extended to allow transactions to be active during dump known as fuzzy dump or online dump * Will study fuzzy checkpointing later 38. Advanced Recovery Algorithm 39. Advanced Recovery Techniques * Support high-concurrency locking techniques, such as those used for B + -tree concurrency control * Operations like B + -tree insertions and deletions release locks early. * They cannot be undone by restoring old values ( physical undo ), since once a lock is released, other transactions may have updated the B + -tree. * Instead, insertions (resp. eletions) are undone by executing a deletion (resp. insertion) operation (known as ratiocinative undo ). * For such operations, undo log records should contain the undo operation to be executed * called synthetical undo logging , in contrast to physical undo logging . * make information is logged physically (that is, new value for each write) even for such operations * Logical redo is very complicated since database state on disk may not be operation consistent 40. Advanced Recovery Techniques (Cont. ) * Operation logging is done as follows * When operation starts, log T i , O j , operation-begin . Here O j is a unique identifier of the operation instance. While operation is executing, normal log records with physical redo and physical undo information are logged. * When operation completes, T i , O j , operation-end , U is logged, where U contains information needed to perform a logical undo information. * If crash/rollback occurs before operation completes * the operation-end log record is not found, and * the physical undo information is used to undo operation. * If crash/rollback occurs after the operation completes * the operation-end log record is found, and in this case * logical undo is performed using U the physical undo information for the operation is ignored. Redo of operation (after crash) still uses physical redo information . 4 1. Advanced Recovery Techniques (Cont. ) * Rollback of transaction T i is done as follows * Scan the log backwards * If a log record T i , X, V 1 , V 2 is found, perform the undo and log a special redo-only log record T i , X, V 1 . * If a T i , O j , operation-end , U record is found * Rollback the operation logically using the undo information U . * Updates performed during roll back are logged just like during normal operation execution. * At the end of the operation rollback, instead of logging an operation-end record, generate a record * T i , O j , operation-abort . Skip all preceding log records for T i until the record T i , O j operation-begin is found 42. Advanced Recovery Techniques (Cont. ) * Scan the log backwards (cont. ) * If a redo-only record is found ignore it * If a T i , O j , operation-abort record is found * skip all preceding log records for T i until the record T i , O j , operation-begi n is found. * Stop the scan when the record T i , start is f ound * Add a T i , abort record to the log * Some points to note * Cases 3 and 4 above can occur only if the database crashes while a transaction is being rolled back. Skipping of log records as in case 4 is important to prevent multiple rollback of the same operation. 43. Advanced Recovery Techniques(Cont,) * The following actions are taken when recovering from system crash * Scan log forward from last checkpoint L record * Repeat history by physically redoing all updates of all transactions, * Create an undo-list during the scan as follows * undo-list is set to L initially * Whenever T i start is found T i is added to undo-list * Whenever T i commit or T i abort is found, T i is deleted from undo-list * This brings database to state as of crash, with committed as well as uncommitted transactions having been redone. Now undo-list contains transactions that are incomplete , that is, have neither committed nor been fully rolled back. 44. Advanced Recovery Techniques (Cont. ) * Recovery from system crash (cont. ) * Scan log backwards, performing undo on log records of transactions found in undo-list . * Transactions are rolled back as described earlier. * When T i start is found for a transaction T i in undo-list , write a T i abort log record. * Stop scan when T i start records have been found for all T i in undo-list * This undoes the set up of incomplete transactions (those with neither commit nor abort log records). Recovery is now complete. 45. Advanced Recovery Techniques (Cont. ) * Checkpointing is done as follows Output all log records in memory to stable storage * Output to disk all modified buffer blocks * Output to log on stable storage a checkpoint L record. * Transactions are not allowed to perform any actions while checkpointing is in progress. * Fuzzy checkpointing allows transactions to progress while the most time consuming parts of checkpointing are in progress * Performed as described on next slide 46. Advanced Recovery Tech niques (Cont. ) * Fuzzy checkpointing is done as follows * Temporarily stop all updates by transactions * Write a checkpoint L log record and force log to stable storage * Note list M of modified buffer blocks Now permit transactions to proceed with their actions * Output to disk all modified buffer blocks in list M * blocks should not be updated while being output * Follow WAL all log records pertaining to a block must be output before the block is output * Store a pointer to the checkpoint record in a fixed position last _ checkpoint on disk * When recovering using a fuzzy checkpoint, start scan from the checkpoint record pointed to by last _ checkpoint * Log records before last _ checkpoint have their updates reflected in database on disk, and need not be redone. * Incomplete checkpoints, where system had crashed while performing checkpoint, are handled safely 47. random memory Recovery Algorithm 48. ARIES * ARIES is a state of the art recovery method * Incorporates numerous o ptimizations to reduce overheads during normal processing and to speed up recovery * The groundbreaking recovery algorithm we studied earlier is modeled after ARIES, but greatly modify by removing optimizations * Unlike the advanced recovery lgorithm, ARIES * Uses log sequence number (LSN) to identify log records * Stores LSNs in pages to identify what updates have already been applied to a database page * physiological redo * unsportsmanlike page table to avoid unnecessary redos during recovery * Fuzzy checkpointing that only records information about grim pages, and does not require dirty pages to be written out at checkpoint time * More approach path up on each of the above 49. ARIES Optimizations * Physiological redo * Affected page is physically identified, action within page can be logical * Used to reduce logging overheads * e. g. hen a record is deleted and all other records have to be moved to fill hole * Physiological redo can log just the record deletion * Physical redo would require logging of old and new values for much of the page * Requires page to be output to disk atomically * golden to achieve with hardware RAID, also supported by some disk systems * Incomplete page output can be detected by checksum techniques, * But extra actions are required for recovery * inured as a media failure 50. ARIES Data Structures * Log sequence number (LSN) identifies each log record * Must be sequentially increasing * Typically an offset from beginning of log point to allow dissolute access * Easily extended to handle multiple log files Each page contains a PageLSN which is the LSN of the last log record whose effects are reflected on the page * To update a page * X-latch the pag, and write the log record * Update the page * Record the LSN of the log record in PageLSN * Unlock page * Page flush to disk S-latches page * Thus page state on disk is operation consistent * Required to support physiological redo * PageLSN is used during recovery to prevent repeated redo * Thus ensuring idempotence 51. ARIES Data Structures (Cont. ) * Each log record contains LSN of previous log record of the same transaction * LSN in log record may be implicit supererogatory redo-only log record called recompense log record (CLR) used to log actions taken during recovery that never need to be undone * Also serve the role of operation-abort log records used in advanced recovery algorithm * Have a field UndoNextLSN to note next (earlier) record to be undone * Records in between would have already been undone * Required to avoid repeated undo of already undone actions LSN TransId PrevLSN RedoInfo UndoInfo LSN TransID UndoNextLSN RedoInfo 52. ARIES Data Structures (Cont. ) * impurePageTable * List of pages in the buffer that have been updated * Contains, for each such page * PageLSN of the page RecLSN is an LSN such that log records before this LSN have already been applied to the page version on disk * Set to current end of log when a page is inserted into dirty page table (just before being updated) * Recorded in checkpoints, helps to minimize redo work * Checkpoint log record * Contains * DirtyPageTable and list of active transactions * For each active transaction, LastLSN, the LSN of the last log record written by the transaction * Fixed position on disk notes LSN of last completed checkpoint log record 53. ARIES Recovery Algorithm * ARIES recovery involves three passes * Analysis pass Determines Which transactions to undo * Which pages were dirty (disk version not up to date) at time of crash * RedoLSN LSN from which redo should start * Redo pass * Repeats history, redoing all actions from RedoLSN * RecLSN and PageLSNs are used to avoid redoing actions already reflected on page * Undo pass * Rolls back all incomplete transactions * Transactions whose abort was complete earlier are not undone * line idea no need to undo these transactions earlier undo actions were logged, and are redone as required 54. ARIES Recovery Ana lysis * Analysis pass * Starts from last complete checkpoint log record Reads in DirtyPageTable from log record * Sets RedoLSN = min of RecLSNs of all pages in DirtyPageTable * In case no pages are dirty, RedoLSN = checkpoint records LSN * Sets undo-list = list of transactions in checkpoint log record * Reads LSN of last log record for each transaction in undo-list from checkpoint log record * Scans forward from checkpoint * .. On next page 55. ARIES Recovery Analysis (Cont. ) * Analysis pass (cont. ) * Scans forward from checkpoint * If any log record found for transaction not in undo-list, adds transaction to undo-list * Whenever an update log record is found If page is not in DirtyPageTable, it is added with RecLSN set to LSN of the update log record * If transaction end log record found, delete transaction from undo-list * Keeps cart track of last log record for each transaction in undo-list * May be needed for later undo * At end of analysis pass * RedoLSN determines where to start redo pass * RecLSN for each page in DirtyPageTable used to minimize redo work * All transactions in undo-list need to be rolled back 56. ARIES Redo Pass * Redo Pass Repeats history by replaying every action not already reflected in the page on disk, as follows * Scans forward from RedoLSN. Whenever an update log record is found * If the page is not in DirtyPageTable or the LSN of the log record is less than the RecLSN of the page in DirtyPageTable, then skip the log record * Otherwise fetch the page from disk.If the PageLSN of the page fetched from disk is less than the LSN of the log record, redo the log record * NOTE if either test is negative the effects of the log record have already appeared on the page. First test avoids even fetching the page from disk 57. ARIES Undo Actions * When an undo is performed for an update log record * Generate a CLR containing the undo action performed (actions performed during undo are logged physicaly or physiologically). * CLR for record n noted as n in figure below * Set UndoNextLSN of the CLR to the PrevLSN value of the update log record * Arrows indicate UndoNextLSN value * ARIES supports partial rollback * Used e. g. o handle deadlocks by rolling back just profuse to release reqd. locks * Figure indicates forward actions after partial rollbacks * records 3 and 4 initially, later 5 and 6, then full rollback 1 2 3 4 4 3 5 6 5 2 1 6 58. ARIES Undo Pass * Undo pass * Performs backward scan on log undoing all transaction in undo-list * Backward scan optimized by skipping unneeded log records as follows * Next LSN to be undone for each transaction set to LSN of last log record for transaction found by analysis pass. * At each step part largest of these LSNs to undo, skip back to it and undo it * After undoing a log record For ordinary log records, set next LSN to be undone for transaction to PrevLSN noted in the log record * For compensation log records (CLRs) set next LSN to be undo to UndoNextLSN noted in the log record * All intervening records are skipped since they would have been undo already * Undos performed as described earlier 59. Other ARIES Features * Recovery Independence * Pages can be recovered independently of others * E. g. if some disk pages fail they can be recovered from a fireman while other pages are being used * Savepoints * Transactions can record savepoints and roll back to a savepoint * Useful for complex transactions Also used to rollback just enough to release locks on deadlock 60. Other ARIES Features (Cont. ) * Fine-grained locking * Index concurrency algorithms that permit tuple level locking on indices can be used * These require logical undo, rather than physical undo, as in advanced recovery algorithm * Recovery optimizations For example * Dirty page table can be used to prefetch pages during redo * Out of order redo is realistic * redo can be postponed on a page being fetched from disk, and performed when page is fetched. * Meanwhile other log records can continue to be processed 61. Remote Backup Systems 62. Remote Backup Systems Remote backup systems provide high availability by allowing transaction processing to continue even if the pristine site is destroyed. 63. Remote Backup Systems (Cont. ) * Detection of failure Backup site must detect when primary winding site has failed * to distinguish primary site failure from link failure maintain several conversation links between the primary and the remote backup. * Transfer of control * To take over control backup site first perform recovery using its copy of the database and all the long records it has received from the primary. * Thus, completed transactions are redone and incomplete transactions are rolled back. When the backup site takes over processing it becomes the new primary * To transfer control back to old primary when it recovers, old primary must receive redo logs from the old backup and apply all updates locally. 64. Remote Backup Systems (Cont. ) * Time to recover To reduce delay in takeover, backup site periodically proceses the redo log records (in effect, performing recovery from previous database state), performs a checkpoint, and can then delete earlier parts of the log. * Hot-Spare configuration permits very fast takeover * Backup continually processes redo log record as they arrive, applying the updates locally. When failure of the primary is detected the backup rolls back incomplete transactions, and is ready to process new transactions. * Alternative to remote backup distributed database with replicated data * Remote backup is faster and cheaper, but less tolerant to failure * more on this in Chapter 19 65. Remote Backup Systems (Cont. ) * Ensure durability of updates by delaying transaction commit until update is logged at backup avoid this delay by permitting lower degrees of durability. * One-safe commit as soon as transactions commit log record is written at primary * Problem updates may not arrive at backup before it takes over . Two-very-safe commit when transactions commit log record is written at primary and backup * Reduces availability since transactions cannot commit if either site fails. * Two-safe proceed as in two-very-safe if both primary and backup are active. If only the primary is active, the transaction commits as soon as is commit log record is written at the primary. * Better availability than two-very-safe avoids problem of lost transactions in one-safe. 66. End of Chapter 67. Block Storage Operations 68. Portion of the Database Log alike to T 0 and T 1 69. conjure of the Log and Database Corresponding to T 0 and T 1 70. Portion of the System Log Corresponding to T 0 and T 1 71. State of System Log and Database Corresponding to T 0 and T 1