LLM Best Practices

Write Amplification

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Overview

Write amplification is physical bytes written divided by logical bytes changed. In PostgreSQL, one UPDATE writes a new heap tuple (mvcc), a new entry in every index on the table unless it qualifies as a HOT (heap-only tuple) update, and WAL records for each modification. Count it when a table takes heavy updates, since each extra index and every WAL record multiplies disk I/O.

Rules

  • Drop unused indexes and use partial indexes. Each index multiplies the cost of every non-HOT write (secondary-index).
  • A HOT update needs no indexed column to change and free space on the same page. Avoid updating indexed columns needlessly, and lower fillfactor on update-heavy tables to leave room.
  • vacuum reclaims dead tuples but adds its own I/O and WAL.
  • Move cold rows out of hot tables (hot-cold-storage).
  • On SSDs high write amplification consumes flash endurance faster. LSM-tree engines (RocksDB, Cassandra) amplify mainly through compaction, which rewrites sorted runs.

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