AWS RDS MySQL Connection Limits Explained

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I have a MySQL database server, db.t3.micro, parameter group is 'default.mysql8.0', max_connections is '{DBInstanceClassMemory/12582880}'

I figured out that it cannot handle more than ~63 connections

According to AWS RDS documentation https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/CHAP_Limits.html#RDS_Limits.MaxConnections

Connection limit for Mysql is calculated using '{DBInstanceClassMemory/12582880}' formula, where DBInstanceClassMemory tier instance memory in bytes.

So for db.t3.micro 1Gb it is 1073741824/12582880 = 85

‘SHOW VARIABLES LIKE "max_connections";’ returns 63

Tier	Memory, GiB	Memory, Bytes	Calculated connection limit	Real connection limit
db.t3.micro	1	1073741824	85	63
db.t3.small	2	2147483648	170	146
db.t3.2xlarge	32	32×1073741824	2720	2634

Why real connections limit is less than the formula result?

Best Answer

So DBInstanceClassMemory is actually TotalMemory- (Memory consumed by OS + other processes)

Amazon documentation clearly says

db.m5.xlarge instance class. DBInstanceClassMemory is 16 GiB, or 17,179,869,184 bytes

but that is not true, and real DBInstanceClassMemory will be less than 16GiB

Related Solutions

Php – AWS RDS connection count

According to this 2011 blog post, the max connections on RDS instances come out this way:

t1.micro: 34 connections ($0.035)
m1.small: 150 connections ($0.115)
m1.large: 640 connections ($0.455)
m1.xlarge: 1263 connections ($0.920)
m2.xlarge: 1441 connections ($0.655)
m2.2xlarge: 2900 connections ($1.315)
m2.4xlarge: 5816 connections ($2.630)

No AWS docs I can find demonstrate this information is current, but other sources suggest if it isn't, it's very close. You can find your instances max connections with:

show variables like '%conn%';

from the mysql terminal, or via an SDK.

Mysql – AWS RDS MySQL remote connection extremely slow

As much I could understand, there is nothing wrong at the ACL or network level, as you are saying that it works sometimes.

What I can see is that you are hitting a limit on the maximum number of processes for Mysql. So, when you connect and issue any command, then if the number of processes allowed to run are saturated, then you have to wait and your request goes into queue. But if you don't get a slot then after that it timeouts.

You can check the same by looking at the graphs which are available with RDS. If the graph is going flat at some value, then my guess is very right.

The total number of connections depends on the instance type you are using. That you can check and set in the DB parameters group option. Search for max_connections.

I am not sure what are the default values for different types of instances, but I am sure this information should be somewhere over the internet.

Best Answer

Related Solutions

Php – AWS RDS connection count

Mysql – AWS RDS MySQL remote connection extremely slow

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