Bug #49171 read_buffer_size is only partially dynamic variable

• Submitted: 27 Nov 2009 20:02
• Modified: 30 Nov 2009 8:14
• Reporter: Andrii Nikitin
• Status: Verified
• Category: MySQL Server: Options
• Severity: S3 (Non-critical)
• Version: 5.1
• OS: Any
• CPU Architecture: Any
• Triage: Triaged: D5 (Feature request)

[27 Nov 2009 20:02] Andrii Nikitin

Description:
on startup mysqld assigns global variable my_default_record_cache_size from global "read_buffer_size" variable.

  my_default_record_cache_size=global_system_variables.read_buff_size;

Later, when "read_buffer_size" is changed, "my_default_record_cache_size" will remain with initial value, which can lead to great misunderstanding.

E.g. bug #44723 will be reproducible even with smallest "read_buffer_size" values if server started with big "read_buffer_size"

How to repeat:
a) Start server before 5.1.35 with "read_buffer_size" = 10M .
b) set global read_buffer_size=128K 
c) reproduce bug #44723 while it claims only "read_buffer_size" greater than 256K can harm.

or see the code.

Suggested fix:
not sure, either make my_default_record_cache_size dynamic (probably is very danger so not acceptable).
or add new variable.

[30 Nov 2009 8:14] Sveta Smirnova

Thank you for the report.

Verified as described reading source code:  my_default_record_cache_size set in ./sql/mysqld.cc and never changes.