Course Database Management Systems - Chapter 6: Database Recovery Techniques - Nguyen Thanh Tung

Nội dung text: Course Database Management Systems - Chapter 6: Database Recovery Techniques - Nguyen Thanh Tung

1. Outline Databases Recovery 1 Purpose of Database Recovery 2 Types of Failure 3 Transaction Log 4 Data Updates 5 Data Caching 6 Transaction Roll-back (Undo) and Roll-Forward 7 Checkpointing 8 Recovery schemes 9 ARIES Recovery Scheme 10 Recovery in Multidatabase System 2
2. Database Recovery 2 Types of Failure The database may become unavailable for use due to • Transaction failure: Transactions may fail because of incorrect input, deadlock, incorrect synchronization. • System failure: System may fail because of addressing error, application error, operating system fault, RAM failure, etc. • Media failure: Disk head crash, power disruption, etc. 4
3. Database Recovery 4 Data Update • Immediate Update: As soon as a data item is modified in cache, the disk copy is updated. • Deferred Update: All modified data items in the cache are written either after a transaction ends its execution or after a fixed number of transactions have completed their execution. • Shadow update: The modified version of a data item does not overwrite its disk copy but is written at a separate disk location. • In-place update: The disk version of the data item is overwritten by the cache version. • Immediate update and deferred update are two main techniques for recovery 6
4. Database Recovery 6 Transaction Roll-back (Undo) and Roll-Forward (Redo) To maintain atomicity, a transaction’s operations are redone or undone. Undo: Restore all BFIMs on to disk (Remove all AFIMs). Redo: Restore all AFIMs on to disk. Database recovery is achieved either by performing only Undos or only Redos or by a combination of the two. These operations are recorded in the log as they happen. 8
5. Database Recovery Roll-back: One execution of T1, T2 and T3 as recorded in the log. A B C D 30 15 40 20 [start_transaction, T3] [read_item, T3, C] * [write_item, T3, B, 15, 12] 12 [start_transaction,T2] [read_item, T2, B] [write_item, T2, B, 12, 18] 18 [start_transaction,T1] [read_item, T1, A] [read_item, T1, D] [write_item, T1, D, 20, 25] 25 [read_item, T2, D] [write_item, T2, D, 25, 26] 26 [read_item, T3, A] system crash * T3 is rolled back because it did not reach its commit point. T2 is rolled back because it reads the value of item B written by T3. 10
6. Database Recovery Write-Ahead Logging When in-place update (immediate or deferred) is used then log is necessary for recovery and it must be available to recovery manager. This is achieved by Write-Ahead Logging (WAL) protocol. WAL states that For Undo: Before a data item’s AFIM is flushed to the database disk (overwriting the BFIM) its BFIM must be written to the log and the log must be saved on a stable store (log disk). For Redo: Before a transaction executes its commit operation, all its AFIMs must be written to the log and the log must be saved on a stable store. 12
7. Database Recovery 7 Checkpointing From time to time (randomly or under some criteria) the database flushes its buffer to database disk to minimize the task of recovery. The following steps defines a checkpoint operation: 1. Suspend execution of transactions temporarily. 2. Force write modified buffer data to disk. 3. Write a [checkpoint] record to the log, save the log to disk. 4. Resume normal transaction execution. During recovery redo or undo is required to transactions appearing after [checkpoint] record. 14
8. Database Recovery 8 Recovery Scheme Deferred Update (No Undo/Redo) The data update goes as follows: 1. A set of transactions records their updates in the log. 2. At commit point under WAL scheme these updates are saved on database disk. After reboot from a failure the log is used to redo all the transactions affected by this failure. No undo is required because no AFIM is flushed to the disk before a transaction commits. 16
9. Database Recovery Deferred Update in a single-user system (a) T1 T2 read_item (A) read_item (B) read_item (D) write_item (B) write_item (D) read_item (D) write_item (D) (b) [start_transaction, T1] [write_item, T1, D, 20] [commit T1] [start_transaction, T2] [write_item, T2, B, 10] [write_item, T2, D, 25]  system crash The [write_item, ] operations of T1 are redone. T2 log entries are ignored by the recovery manager. (T2 is not committed.) 18
10. Database Recovery Deferred Update with concurrent users (a) T1 T2 T3 T4 read_item (A) read_item (B) read_item (A) read_item (B) read_item (D) write_item (B) write_item (A) write_item (B) write_item (D) read_item (D) read_item (C) read_item (A) write_item (D) write_item (C) write_item (A) (b) [start_transaction, T1] [write_item, T1, D, 20] [commit, T1] [checkpoint] [start_transaction, T4] [write_item, T4, B, 15] [write_item, T4, A, 20] [commit, T4] [start_transaction T2] [write_item, T2, B, 12] [start_transaction, T3] [write_item, T3, A, 30] [write_item, T2, D, 25]  system crash T2 and T3 are ignored because they did not reach their commit points. T4 is redone because its commit point is after the last checkpoint. 20
11. Database Recovery Recovery Techniques Based on Immediate Update Undo/No-redo Algorithm In this algorithm AFIMs of a transaction are flushed to the database disk under WAL before it commits. For this reason the recovery manager undoes all transactions during recovery. No transaction is redone. It is possible that a transaction might have completed execution and ready to commit but this transaction is also undone. 22
12. Database Recovery Recovery Techniques Based on Immediate Update Undo/Redo Algorithm (Concurrent execution) Recovery schemes of this category applies undo and also redo to recover the database from failure. In concurrent execution environment a concurrency control is required and log is maintained under WAL. Commit table records transactions to be committed and active table records active transactions. To minimize the work of the recovery manager, checkpointing is used. The recovery performs: 1. Undo of a transaction if it is in the active table. 2. Redo of a transaction if it is in the commit table. 24
13. Database Recovery Shadow Paging To manage access of data items by concurrent transactions two directories (current and shadow) are used. The directory arrangement is illustrated below. Here a page is a data item. Current Directory Database disk blocks (pages) Shadow Directory (after updating pages 2, 5) (not updated) Page 5 (old) 1 Page 1 1 2 Page 4 2 3 Page 2 (old) 3 4 Page 3 4 5 Page 6 5 6 Page 2 (new) 6 Page 5 (new) 26
14. Database Recovery The ARIES Recovery Algorithm The ARIES recovery algorithm consists of three steps: 1. Analysis: step identifies the dirty (updated) pages in the buffer and the set of transactions active at the time of crash. The appropriate point in the log where redo is to start is also determined. 2. Redo: necessary redo operations are applied. 3. Undo: log is scanned backwards and the operations of transactions active at the time of crash are undone in reverse order. 28
15. Database Recovery The ARIES Recovery Algorithm The Log and Log Sequence Number (LSN) A log record stores: 1. Previous LSN of that transaction: It links to the log record of each transaction. It is like a back pointer that points to the previous record of the same transaction. 2. Transaction ID 3. Type of log record. For a write operation the following additional information is logged: 4. Page ID for the page that includes the item 5. Length of the updated item 6. Its offset from the beginning of the page 7. BFIM of the item 8. AFIM of the item 30
16. Database Recovery The ARIES Recovery Algorithm The Transaction table and the Dirty Page table For efficient recovery following tables are also stored in the log during checkpointing: Transaction table: Contains an entry for each active transaction, with information such as transaction ID, transaction status and the LSN of the most recent log record for the transaction. Dirty Page table: Contains an entry for each dirty page in the buffer, which includes the page ID and the LSN corresponding to the earliest update to that page. 32
17. Database Recovery The ARIES Recovery Algorithm The following steps are performed for recovery 1. Analysis phase: Start at the begin_checkpoint record and proceed to the end_checkpoint record. Access transaction table and dirty page table are appended to the end of the log. Note that during this phase some other log records may be written to the log and transaction table may be modified. The analysis phase compiles the set of redo and undo to be performed and ends. 2. Redo phase: Starts from the point in the log up to where all dirty pages have been flushed, and move forward to the end of the log. Any change that appears in the dirty page table is redone. 3. Undo phase: Starts from the end of the log and proceeds backward while performing appropriate undo. For each undo it writes a compensating log record in the log. The recovery completes at the end of undo phase. 34
18. Database Recovery 10 Recovery in multidatabase system A multidatabase system is a special distributed database system where one node may be running relational database system under Unix, another may be running object-oriented system under Window and so on. A transaction may run in a distributed fashion at multiple nodes. In this execution scenario the transaction commits only when all these multiple nodes agree to commit individually the part of the transaction they were executing. This commit scheme is referred to as “two-phase commit” (2PC). If any one of these nodes fails or cannot commit the part of the transaction, then the transaction is aborted. Each node recovers the transaction under its own recovery protocol. 36