Background writer cannot keep up with I/O demand

When buffers_backend is significantly higher than buffers_clean and buffers_checkpoint combined, the background writer cannot keep up with dirty buffer writes. Backends are forced to write their own buffers, causing I/O bottlenecks.

Increase bgwriter_lru_maxpages and bgwriter_lru_multiplier configuration parameters. Monitor pg_stat_bgwriter for buffers_backend vs buffers_clean ratios. If checkpoints_req is much higher than checkpoints_timed, increase checkpoint_timeout or max_wal_size.