Shadow paging

Shadow Paging

• This technique does not require LOG in single user environment

• In multi-user may need LOG for concurrency control method

• Shadow paging considers

• The database is partitioned into fixed-length blocks referred to as PAGES.

• Page table has n entries – one for each database page.

• Each contain pointer to a page on disk (1 to 1st page on database and so on…).

The idea is to maintain 2 pages tables during the life of transaction.

• The current page table
• The shadow page table

When transaction starts, both page tables are identical

• The shadow page table is never changed over the duration of the transaction.
• The current page table may be changed when a transaction performs a write operation.
• All input and output operations use the current page table to locate database pages on disk.

Advantages

• No Overhead for writing log records.
• No Undo /  No Redo algorithm.
• Recovery is faster.
  

Disadvantages

• Data gets fragmented  or scattered.
• After every transaction completion database pages containing old version of modified data need to be garbage collected.
• Hard to extend algorithm to allow transaction to run concurrently.
  

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