1 / 14

Recovery Basics

Recovery Basics. Types of Recovery. Catastrophic – disk crash Backup from tape; redo from log Non-catastrophic: inconsistent state Undo some operations Redo other ops. Update Times. Deferred update After transaction finish: write to log, then DB No-undo/redo Immediate update

lyris
Download Presentation

Recovery Basics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Recovery Basics

  2. Types of Recovery • Catastrophic – disk crash • Backup from tape; redo from log • Non-catastrophic: inconsistent state • Undo some operations • Redo other ops

  3. Update Times • Deferred update • After transaction finish: write to log, then DB • No-undo/redo • Immediate update • Write to log and DB before transaction finished • Undo/redo • If all transactions are written to DB before commit: undo/no-redo

  4. Recovery Concepts • Cache of DB disk pages in RAM • Dirty bit • In-place vs. shadowing • Before image (BFIM); after image (AFIM) • Log • Write-ahead logging (WAL) • Undo entries • Contains BFIM info • Redo entries • Contains AFIM info

  5. Rollback • Roll back effect of aborted transaction • Replace current info. By BFIM info. From the log • May cascade to other transactions that have read modified items

  6. All recovery operations must be idempotent: can be executed many times and have the effect of only being executed once • Implies that log values are absolute and never relative.

  7. Deferred update • A transaction cannot change the database until it reaches its commit point. • A transaction does not reach its commit point until all its update operations are recorded in the log ad the log is force-written to disk. • No-UNDO/REDO

  8. Immediate Update • database can be updated immediately without waiting for the transaction to commit, provided the log is written to disk first. • If all updates of a transaction are recorded in the database on disk before the transaction commits: UNDO/NO-REDO • If the transaction can commit before all its changes are written to the database: UNDO/REDO

  9. Shadow Paging • At the beginning of a transaction the page table is copied and stored • Write updates write a new page to disk, never modifying pages • In recovery, free the modified pages, and restore the page table. • NO-UNDO/NO-REDO • Need checkpoints and logs for concurrency control • Disadvantages: • scatters data pages • shadow page table management overhead • garbage collection overhead

  10. Two phase Commit • Need a global recovery manager • Phase 1: • each DB signals that it has finished its part of the transaction • coordinator sends “prepare to commit” message • DB’s force write logs to disk, return message “ready to commit”/”cannot commit” • Coordinator can time out and assum problem • Phase 2: • If all DB’s reply “OK”, coordinator sends commit. Recovery is possible • If some DB is “not OK”, coordinator sends “Roll back” • Failure in phase 1 requires roll back • Failure in phase 2 means a successful transaction can recover and commit.

  11. Catastrophic Failure • Backup: • whole database & log • incremental snapshots of log • Cannot cover every conceivable situation • Uptime constraints: need to freeze database as they are being backed up

  12. ARIES • Analysis • identifies dirty buffer pool pages, and active transactions at time of crash • Redo • repeats all actions in the log starting from last checkpoint • Undo • Undoes the actions of transactions that did not commit

  13. ARIES principles • WAL • Repeating History during Redo • Logging changes during Undo

  14. For more Information on ARIES see Chpt. 18 in Ramakrishnan’s DBMS book.

More Related