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I run a fairly standard LAMP stack with an Amazon RDS database on a t2.small instance. Today I was alerted through Website monitoring that the database seemed to have "locked up" completely. The symptoms I found were:

  • No queries running for a long time when running

    show processlist
    
  • Apache web server not busy at all

Reviewing the logs configured in Cloudwatch showed at the time of the outage I found that all throughput metrics went to near 0. Connections were up to a spike of 17. The connection limit is configured as per default to InstanceMemory/12MB, so that's about 160 connections. Freeable memory went from 264MB to 234MB.

CPU Credit Balance graph for the RDS instance shows CPU is at 250 consistently, web server is very high as well. There is no specific traffic I can point to, I have picked some urls out of the web server logs and counted request in that hour and compared to other hours. Nothing that sticks out.

How could I diagnose this further?

SHOW VARIABLES

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  • That's less than 2GB of RAM? I hope that innodb_buffer_pool_size is no more than 600M. Please provide us the MySQL configuration settings. What was the value of Max_used_connections? What value do you use for autocommit? – Rick James Oct 7 '17 at 23:42
  • @RickJames Have added SHOW VARIABLES; buffer pool size if 1.4GB, autocommit is ON. Max connections was 29, so that is 348MB+1400 for innodb buffer gets very close to 2GB. This seems like to solve the problem for me. Please add your answer. – automatem Oct 8 '17 at 21:16
  • @automatem Please post results of SHOW GLOBAL VARIABLES LIKE 'threads_%'. – Wilson Hauck Oct 10 '17 at 2:44
  • 1.4G/2.0G is much too crowded to allow for code, other buffers/tables/etc, operating system, etc. Swapping is teeeeerrrrible for MySQL performance. You are swapping. – Rick James Oct 11 '17 at 5:12
2

Implementing Rick James' suggestion of 600MB for innodb_buffer_pool_size will help.

Changing:

innodb_buffer_pool_instances = 2  # from 8

...could also be helpful.

max_allowed_packet at 32M would not take many active connections to stress RAM capacity of 2GB.

See the 5.7 documentation:

Internally net_buffer_length is dynamically allowed to grow during a session to this limit. Default is 1MB, using session variables can be used to manage specific needs, when required.

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