This technique does not require LOG in single user environment
Advantages
- 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.
- 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.
