Shadow Paging

Shadow Paging

• Shadow paging is a recovery scheme in which database is considered to be made up of number of fixed size disk pages.

• A directory or a page table is constructed with n number of pages where each i^th page points to the i^th database page on the disk.

• The directory can be kept in the main memory.

• When a transaction begins executing, the current directory-whose entries point to the most recent or current database pages on disk-is copied into a another directory called shadow directory.

• The shadow directory is then saved on disk while the current directory is used by the transaction.

• During the execution of transaction, the shadow directory is never modified.

• When a write operation is to be performed then the new copy of modified database page is created but the old copy of database page is never overwritten. This newly created database page is written somewhere else.

• The current directory will point to newly modified web page and the shadow page directory will point to the old web page entries of database disk.

• When the failure occurs then the modified database pages and current directory is discarded.

• The state of database before the failure occurs is now available through the shadow directory and this state can be recovered using shadow directory pages.