fsync and Crash-Safe Write Ordering

Write a C program that simulates crash-safe write ordering for a simple database update.

Requirements:

• Simulate updating a database with two pages: a data page (page 5) and an index page (page 2) that points to it
• Simulate three write strategies: Strategy A (unsafe): write index page, write data page, no fsync. Simulate a crash after index write. Show that the index points to garbage. Strategy B (safe but slow): write data page, fsync, write index page, fsync. Show it is safe but requires 2 fsyncs. Strategy C (WAL): append log record "update page 5, then update page 2", fsync log, write data page, write index page. Simulate crash after log fsync. Show recovery replays the log.
• For each strategy, print the sequence of operations and whether data is consistent after crash
• Calculate the total fsync count for each strategy
• Print a note about group commit: how batching 10 transactions into one fsync reduces fsync count from 10 to 1
• Explain why fsync ordering matters more than write ordering (the OS can reorder writes)