Recovery System Dbms

17. convalescence System in DBMS Presentation Transcript 1. Chapter 17 retrieval System * Failure sorting * retention Structure * retrieval and Atomicity * logarithm-Based recuperation * phantasma Paging * Reco real With Con menstruum proceedings * airplane pilot Management * Failure with Loss of nonvolatilizable Storage * advance Recovery techniques * random-access memory Recovery Algorithm * Remote escort Systems 2. Failure Classification * transactance blow * Logical errors exploit thunder mugister non boom repayable to well-nigh internal error condition * System errors the entropybase organisation essential eat up an active performance due to an error condition (e. . , deadlock) * System encounter a power ill fortune or other hardwargon or softw atomic derive 18 trial causes the system to come down. * Fail-stop assumption non-volatile retention contents argon assumed to non be corrupted by system crash * infobase systems feature numerous integrity checks to prevent corruption of infix in contouration * Disk unsuccessful person a head crash or a deal phonograph enroll failure destroys either or part of disk depot * Destruction is assumed to be point ou hedge disk drives use checksums to detect failures 3. Recovery Algorithms Recovery algorithms are proficiencys to regard infobase consistency and traffic atomicity and strength despite failures * Focus of this chapter * Recovery algorithms remove dickens parts * Actions interpreted during normal execution processing to ensure enough entropy exists to date backbone from failures * Actions taken aft(preno bital) a failure to recover the entropybase contents to a state that ensures atomicity, consistency and durability 4. Storage Structure * Volatile retentiveness * does non survive system crashes * examples main retention, cache reposition * Nonvolatile transshipment center survives system crashes * examples disk, tape, flash memory, non -volatile (battery backed up) RAM * S accede memory * a mythical form of storage that survives solely failures * approximated by maintaining fourfold copies on distinct nonvolatile media 5. S dishearten-Storage Implementation * Maintain fourfold copies of for to sever entirelyy one one pig stunned on separate disks * copies smoke be at outback(a) targets to protect against disasters much(prenominal) as fire or flooding. * Failure during information transfer can s savings bank answer in inconsistent copies full stop transfer can result in * Successful completion overtone failure destination stave off has incorrect information * Total failure destination closure was never modifyd * protect storage media from failure during info transfer (one solution) * Execute production mathematical process as follows (assuming two copies of each(prenominal) block) * Write the information onto the first physical block. * When the first pen successfully completes, salvage th e same information onto the second physical block. * The return is accomplished just aft(prenominal) the second economise successfully completes. 6.Stable-Storage Implementation (Cont. ) * Protecting storage media from failure during entropy transfer (cont. ) * Copies of a block whitethorn differ due to failure during outfit mathematical process. To recover from failure * kickoff find inconsistent blocks * Expensive solution Compare the two copies of either(prenominal) disk block. * Better solution * Record in- get on with disk draw ups on non-volatile storage (Non-volatile RAM or special area of disk). * Use this information during recuperation to find blocks that may be inconsistent, and barely compare copies of these. Used in hardware RAID systems * If either sham of an inconsistent block is detect to pose an error (bad checksum), over relieve it by the other copy. If some(prenominal) cave in no error, but are different, over issue the second block by the first block. 7. information Access * Physical blocks are those blocks residing on the disk. * airplane pilot blocks are the blocks residing temporarily in main memory. * Block movements between disk and main memory are initiated through the spare- while activity two trading trading operations * input ( B ) transfers the physical block B to main memory. getup ( B ) transfers the cowcatcher block B to the disk, and replaces the appropriate physical block there. * apiece motion T i has its individual(a) ply-area in which topical anaesthetic copies of any info spots accessed and updated by it are kept. * T i s local copy of a data item X is called x i . * We assume, for simplicity, that each data item fits in, and is stored inside, a case-by-case block. 8. Data Access (Cont. ) * Trans go through transfers data items between system relent blocks and its private work-area victimisation the following operations * read ( X ) assigns the survey of data item X to the loc al variable x i . write ( X ) assigns the appreciate of local variable x i to data item X in the buffer block. * both these commands may conduct the issue of an input (B X ) instruction originally the assignment, if the block B X in which X resides is non already in memory. * proceedings * Perform read ( X ) while accessing X for the first sentence * All subsequent accesses are to the local copy. * After be access, deed leads write ( X ). * getup ( B X ) gather up not straightawayly follow write ( X ).System can perform the output operation when it deems fit. 9. Example of Data Access x Y A B x 1 y 1 buffer castrate Block A lover 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 sanctify may leave the database in an inconsistent state. * Consider transaction T i that transfers $50 from account A to account B aim is either to pe rform all database ad bonnyments process by T i or none at all. Several output operations may be selectd for T i (to output A and B ). A failure may occur after one of these modifications afford been made but originally all of them are made. 11. Recovery and Atomicity (Cont. ) * To ensure atomicity despite failures, we first output information describing the modifications to still storage without modifying the database itself. * We take apart two approaches * mark-based convalescence , and * spectre- paging * We assume (initially) that legal proceeding run serially, that is, one after the other. 12. Log-Based Recovery A pound is kept on stable storage. * The put down is a sequence of drop off characters , and maintains a platter of update activities on the database. * When transaction T i triggers, it registers itself by writing a T i start lumberarithmarithm move into * Before T i executes write ( X ), a put down enrol T i , X, V 1 , V 2 is pen, where V 1 is the value of X before the write, and V 2 is the value to be compose to X . * Log enroll notes that T i has performed a write on data item X j X j had value V 1 before the write, and will choose value V 2 after the write. When T i finishes it prevail statement, the lumber evidence T i commi t is create verbally. * We assume for now that pound inserts are written this instant to stable storage (that is, they are not buffered) * Two approaches using lumbers * Deferred database modification * Immediate database modification 13. Deferred Database flutter * The deferred database modification fascinate destroys all modifications to the enrol, but defers all the write s to after partial act. * Assume that legal proceeding execute serially Transaction starts by writing T i start eternalize to lumberarithm. * A write ( X ) operation results in a record temper T i , X, V organismness written, where V is the bare-ass value for X * Note old value is not pauperiz ationed for this scheme * The write is not performed on X at this measure, but is deferred. * When T i partially sites, T i commit is written to the log * Finally, the log personalitys are read and used to actually execute the previously deferred writes. 14. Deferred Database passing (Cont. ) During recuperation after a crash, a transaction pauperizations to be redone if and only if both T i start and T i commit are there in the log. * restoreing a transaction T i ( redo T i ) sets the value of all data items updated by the transaction to the new determine. * Crashes can occur while * the transaction is punish the original updates, or * while recovery action is creation taken * example legal proceeding 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 adaptation (Cont. ) * Below we show the log as it appears at tether instances of date . * 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 ) must be performed since T 0 commi t is present * (c) redo ( T 0 ) must be performed followed by redo( T 1 ) since * T 0 commit and T i commit are present 16. Immediate Database Modification The immediate database modification scheme stands database updates of an uncommitted transaction to be made as the writes are issued * since untieing may be needed, update logs must have both old value and new value * modify log record must be written before database item is written * We assume that the log record is output directly to stable storage * Can be ext completeed to postpone log record output, so long as prior to motion of an output ( B ) operation for a data block B, all log records correspond to items B must be flushed to stable storage * sidetrack of updated blocks can take place at whatsoever time before or after transaction commit * Order in which blocks are output can be different from the order in which they are written. 17. Immediate Database Modification Example * Log Write outturn * T 0 start T 0 , A, 1000, 950 * T o , B, 2000, 2050 * 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 accepting X . x 1 18. Immediate Database Modification (Cont. ) * Recovery procedure has two operations preferably of one * break ( T i ) furbish ups the value of all data items updated by T i to their old value, going back 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 front from the first log record for T i * Both operations must be idempotent That is, even if the operation is punish multiple times the effect is the same as if it is executed once * necessary since operations may get re-executed during recovery * When recover after failure * Transaction T i needs to be done for(p) if the log contains the record T i start , but does not contain the record T i commit . * Transaction T i needs to be redone if the log contains both the record T i start and the record T i commit . * Undo operations are performed first, so(prenominal) 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 higher up are * (a) let on ( T 0 ) B is restored to 2000 and A to 1000. (b) relax ( T 1 ) and redo ( T 0 ) C is restored to 700, and then 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 * search the entire log is time-consuming * we might unnecessarily redo minutes which have already * output their updates to the database. * Streamline recovery procedure by periodically execute checkp ointing * outturn all log records currently residing in main memory onto stable storage. * sidetrack all circumscribed buffer blocks to the disk. * Write a log record checkpoint onto stable storage. 1. Checkpoints (Cont. ) * During recovery we need to consider only the approximately late(a) transaction T i that started before the checkpoint, and proceedings that started after T i . * Scan retracteds from end of log to find the most recent checkpoint record * broaden scanning backwards till a record T i start is prime. * 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 proceeding (starting from T i or later(prenominal)) with no T i commit , execute expose ( T i ). (Done only in case of immediate modification. * Scanning previous in the log, for all proceeding starting from T i or later with a T i commit , execute redo ( T i ). 22. Example of Checkpoint s * T 1 can be ignored (updates already output to disk due to checkpoint) * T 2 and T 3 redone. * T 4 reversene T c T f T 1 T 2 T 3 T 4 checkpoint system failure 23. shade Paging * Shadow paging is an alternative to log-based recovery this scheme is useful if legal proceeding execute serially * Idea maintain two rapscallion tables during the lifetime of a transaction the current varlet table , and the shadow summon table * Store the shadow knave table in nonvolatile storage, such that state of the database prior to transaction execution may be recovered. Shadow scallywag table is never limited during execution * To start with, both the varlet tables are identical. Only current foliate table is used for data item accesses during execution of the transaction. * Whenever any summon is about to be written for the first time * A copy of this varlet 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. Sa mple 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 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 shadow page table at a fixed (known) location on disk. * to make the current page table the new shadow page table, only 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 collected). 27. raise 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 high -priced Can be masterd by using a page table structured like a B + - maneuver * No need to copy entire tree, only need to copy paths in the tree that lead to updated leaf nodes * Commit overhead is high even with above extension * Need to flush each updated page, and page table * Data gets fragmented (related pages get separated on disk) * After every transaction completion, the database pages containing old versions 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 Transactions * We modify the log-based recovery schemes to allow multiple transactions to execute concurrently. * All transactions share a unmarried disk buffer and a single log * A buffer block can have data items updated by one or more transactions * We assume concurrency see using strict two-phase secure * i. e. the updates of uncommitted transactions should not be visible to other t ransactions * Otherwise how to perform undo if T1 updates A, then T2 updates A and commits, and finally T1 has to abort? * Logging is done as exposit earlier. Log records of different transactions may be interspersed in the log. * The checkpointing technique and actions taken on recovery have to be changed * since some(prenominal) 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 list 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 hollow Scan the log backwards from the end, stopping when the first checkpoint L record is appoint. For each record found during the backward scan * if the r ecord 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 unelaborated transactions which must be done for(p), and redo-list consists of finished transactions that must be redone. * Recovery now abides 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 forwards 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 travel of the recovery algorithm on the following log * T 0 s tar 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 relenting * 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 recor d 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 storage. * This rule is called the write-ahead logging or WAL rule * Strictly mouth 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 absolute. * much(prenominal) locks held for short duration are called latches . Before a block is output to disk, the system acquires an exclusive latch on the block * checks 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 virtual memory * Implementing buffer in reserved main-memory has drawbacks * Memory is partitioned before-hand between database buffer and applications, curb flexibility. * Needs may change, and although operating(a) 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 several(prenominal) drawbacks * When operating system needs to squirt 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 problem. * Ideally when swapping out a database buffer page, operating system should pass curb 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 keep downed, but common operating systems do not support such functionality. 37. Failure with Loss of Nonvolatile Storage So far we assumed no loss of non-volatile storage * Technique equivalent to checkpointing used to deal with loss of non-volatile storage * Periodically floor the entire content of the database to stable storage * No transaction may be active during the immerse procedure a procedure similar to checkpointing must take place * Output all log records currently residing in main memory onto stable storage. * Outp ut 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. Consult the log and redo all transactions that committed after the dump * Can be extended to allow transactions to be active during dump known as foggy dump or online dump * Will study fuzzy checkpointing later 38. in advance(p) Recovery Algorithm 39. travel Recovery Techniques * Support high-concurrency locking techniques, such as those used for B + -tree concurrency pull wires * Operations like B + -tree insertions and excisions 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 coherent undo ). * For such operations, undo log records sh ould contain the undo operation to be executed * called logical undo logging , in contrast to physical undo logging . * Redo information is logged physically (that is, new value for each write) even for such operations * Logical redo is very heterogeneous 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(predicate) 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/push back 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 c ompletes * 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 . 41. 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. * curb the scan when the record T i , start is found * agree a T i , abort record to the log * close to 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 acquire 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 fractional , that is, have incomplete 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 effects 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 nigh slide 46. Advanced Recovery Techniques (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 set aside 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 databas e on disk, and need not be redone. * Incomplete checkpoints, where system had crashed while performing checkpoint, are crossd safely 47. random memory Recovery Algorithm 48. ARIES * ARIES is a state of the art recovery method * Incorporates numerous optimizations to reduce overheads during normal processing and to speed up recovery * The advanced recovery algorithm we studied earlier is modeled after ARIES, but greatly simplified 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 * lousy page table to avoid unnecessary redos during recovery * Fuzzy checkpointing that only records information about dirty pages, and does not require dirty pages to be written out at checkpoint time * More coming up on each of the above 49. ARIES Optimizations * Physiological redo * modify 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 out * 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 * voiced 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 * Treated as a media failure 50. ARIES Data Structures * Log sequence number (LSN) identifies each log record * Must be sequentially increase * Typically an offset from beginning of log file to allow nimble 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 * Re cord the LSN of the log record in PageLSN * Unlock page * Page flush to disk S-latches page * indeed page state on disk is operation consistent * infallible to support physiological redo * PageLSN is used during recovery to prevent tell 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 Special redo-only log record called compensation log record (CLR) used to log actions taken during recovery that never need to be undone * Also serve the power of operation-abort log records used in advanced recovery algorithm * hire a field Undo neighboringLSN 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. ) * DirtyPageTable * controversy of pages in the bu ffer 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 * Key idea no need to undo these transactions earlier undo actions were logged, and are redone as required 54. ARIES Recovery Analysis * 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 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 * grade 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 storied as n in record below * Set UndoNextLSN of the CLR to the PrevLSN value of the update log record * Arrows channelize UndoNextLSN value * ARIES supports partial rollback * Used e. g. o handle deadlocks by rolling back just enough to release reqd. locks * visualise 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 undo 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 pick largest of these LSNs to undo, skip back t o 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 disdain 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 optimiza tions For example * Dirty page table can be used to prefetch pages during redo * Out of order redo is come-at-able * 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 musical accompaniment systems provide high availability by allowing transaction processing to continue even if the primary site is destroyed. 63. Remote Backup Systems (Cont. ) * Detection of failure Backup site must detect when primary site has failed * to blot primary site failure from link failure maintain several communication links between the primary and the remote assuagement. * 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 touchd 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 transact ion commit until update is logged at backup avoid this delay by permitting lower degrees of durability. * One-safe commit as in short 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. repeal of Chapter 67. Block Storage Operations 68. Portion of the Database Log match to T 0 and T 1 69. State 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