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Defaults and tuning guidance for 8.4

MySQL 8.4 updates several server defaults to align with modern CPUs, memory sizes, and SSD/NVMe storage. An in-place upgrade that blindly reuses an 8.0-era my.cnf may miss out on these improvements or cause unexpected performance behaviors. Review and re-evaluate your configuration on 8.4, or generate a new config, rather than carrying old settings forward.

Notable InnoDB default changes

InnoDB System Variable Name New Default (8.4) Previous Default (8.0)
innodb_adaptive_hash_index OFF ON
innodb_change_buffering none all
innodb_doublewrite_files 2 innodb_buffer_pool_instances * 2
innodb_doublewrite_pages 128 4
innodb_flush_method on Linux O_DIRECT if supported, otherwise fsync fsync
innodb_io_capacity 10000 200
innodb_log_buffer_size 67108864 (64 MiB) 16777216 (16 MiB)
innodb_numa_interleave ON OFF
temptable_max_ram 3% of total memory (1–4 GiB range) 1073741824 (1 GiB)
innodb_parallel_read_threads available logical processors / 8 (min 4) 4

Why these changes matter:

  • Higher innodb_io_capacity leverages SSD/NVMe for IO-bound workloads; legacy spinning disks may need a lower value.
  • Larger innodb_log_buffer_size reduces redo flush frequency—helpful for write-heavy workloads.
  • innodb_adaptive_hash_index default OFF favors predictability; the adaptive hash index can become a contention source under concurrency.
  • innodb_numa_interleave defaults ON to reduce memory imbalance on multi-socket systems.
  • innodb_change_buffering set to none reduces overhead for modern storage that handles random writes efficiently.
  • innodb_doublewrite_pages increased to 128 improves doublewrite performance on fast storage.

Configuration review checklist

Use this to adapt an 8.0 configuration to 8.4:

  • Remove overrides that merely reassert old 8.0 defaults unless they are proven necessary.
  • Re-evaluate IO settings (innodb_io_capacity, flush method) based on storage type and observed latency.
  • Confirm redo/undo settings and log buffer meet current write patterns.
  • Validate parallel read threads relative to CPU topology and workload.
  • Generate a fresh config for 8.4 when possible; only reapply carefully justified overrides.

Practical evaluation steps

  • Benchmark with your workload: establish a baseline on 8.0, then restore to 8.4 and run the same tests.
  • Compare Performance Schema metrics and wait events for regressions or new hotspots.
  • Adjust a single variable at a time; document changes and their impacts.

Further reading