Intermediate LevelPostgreSQL
How does Write-Ahead Logging (WAL) and Checkpointing guarantee durability and crash recovery?
❓ Interview Prompt: Explain Write-Ahead Logging (WAL), REDO log records, `shared_buffers` dirty pages, Checkpoints (`max_wal_size`), and Crash Recovery.
Write-Ahead Logging (WAL) guarantees ACID Durability and Atomicity in PostgreSQL:
WAL Mechanics:
1
All table/index changes are written to append-only WAL files on disk BEFORE dirty data pages are updated in 'shared_buffers' RAM.
2
Checkpoints: Periodic events that flush all dirty RAM buffers to physical storage disk, truncating old WAL logs.
3
Crash Recovery: Upon power failure, PostgreSQL replays WAL records from the last valid Checkpoint to restore database consistency!
Code Implementation Example
sql
-- View current WAL write LSN (Log Sequence Number) position
SELECT pg_current_wal_lsn();
-- Force a manual Checkpoint flush to disk
CHECKPOINT;💡Inspecting current WAL LSN position and issuing manual Checkpoints.
🎯 Interview Key Takeaway
WAL guarantees data durability by logging changes to disk before dirty shared buffers are written to data files.
#WAL#Checkpoint#Durability#Crash Recovery#Intermediate
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