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Shadow paging is a database recovery technique designed to maintain database consistency in the event of transaction failures, system crashes, or other interruptions. It provides a mechanism for recovering the database without requiring a traditional log of every update. The basic idea is to maintain two page tables: a shadow page table and a current page table. The shadow page table points to the stable versions of database pages, while the current page table is used by the active transaction to make changes. Shadow paging is based on the principle of maintaining an unchanged copy of the database structure until a transaction successfully completes. A database is normally divided into fixed-size pages, and a page table contains pointers to the physical locations of these pages. At the beginning of a transaction, the system creates a shadow page table that contains pointers to the original database pages. This shadow page table is stored on stable storage and is not modified during the transaction. A current page table is also maintained in main memory or on storage. Initially, both page tables point to the same physical database pages. When a transaction modifies a page, the original page is not overwritten. Instead, a new copy of that page is created at another location on disk, and the current page table is changed so that it points to the newly created page. The shadow page table continues to point to the original, unchanged page. This process is known as copy-on-write. Consequently, the shadow page table always represents the database state before the transaction began, while the current page table represents the database state after the transaction's modifications. Suppose a transaction modifies pages P1 and P2. Initially, both the shadow and current page tables point to the original copies of P1 and P2. When P1 is updated, a new copy P1' is created and the current page table points to P1'. The shadow page table still points to P1. Similarly, when P2 is modified, a new copy P2' is created and the current page table points to it. Thus, the original database remains available through the shadow page table throughout transaction execution. When the transaction completes successfully, the system must make the new database state permanent. This is _________ ____ _________ ___ __________ _________.
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