read_buffer_size
INFO
Rule IDs: mem_006
Overview
- Purpose
- Documented in the MySQL 8.4 manual as a server system variable (scope: Both). Purpose and semantics are described at the linked manual page.
- Dynamic (MySQL 8.4 reference)
- MySQL 8.4 marks this variable as dynamic (
Dynamic= Yes). Runtime changes useSET GLOBAL(global scope) orSET SESSION(session scope) — confirm syntax and persistence (SET PERSIST) in the manual. - Default value
- 131072 (128 KB) (MySQL 8.4)
- Version and product notes
- MariaDB and Percona Server may use different names, defaults, or dynamic behavior; verify their documentation.
- Documentation
- https://dev.mysql.com/doc/refman/8.4/en/server-system-variables.html#sysvar_read_buffer_size
- Other vendors
What is checked
Rules that reference this variable, with their severity and what each rule detects:
- INFO
mem_006: Keep read_buffer_size at 128KB-256KB unless benchmarking shows clear improvement with larger values.
Tuning guidance
- Recommended actions:
- Keep read_buffer_size at 128KB-256KB unless benchmarking shows clear improvement with larger values.
- Trade-offs: Per-thread buffers are allocated per-session on demand. Large values waste memory when multiplied by max_connections. Small values cause disk spills for complex queries. Tune based on query profiles, not guesswork.
Example
Consult the manual for whether SET GLOBAL, SET PERSIST, or SET SESSION applies.