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The main reasons production DBMSs avoid shadow paging in favour of WAL are that shadow paging
Acannot represent transactions with multiple writes
Bis forbidden by the SQL standard
Crequires SSD-only storage
Dscales poorly with concurrent transactions
Answer & Solution
Correct answer: D. scales poorly with concurrent transactions
Shadow paging is conceptually clean (commit by atomic page-table swap) but hard to combine with concurrent transactions touching the same pages, and the page table grows large when many pages are modified. Garbage collection of orphaned shadow pages is also non-trivial. WAL avoids all of these issues, which is why ARIES-style logging won.
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