Expand description
Cache / buffer collapse residuals.
Two channels per source:
- hit-ratio drop —
expected_hit_ratio − observed_hit_ratio, positive when the cache is failing. - I/O wait amplification —
observed_io_wait_seconds / baseline_io_wait_seconds − 1.0, positive when I/O is slower than the rolling baseline.
In PostgreSQL 16+ both are first-class via pg_stat_io.hits and
pg_stat_io.read_time (fact #44). In SQL Server they are reachable via
sys.dm_os_buffer_descriptors plus Query Store I/O wait stats. In Oracle
they are in V$SEGMENT_STATISTICS plus V$SYSTEM_EVENT.