After more googling it appears that max_pool_size is a maximum limit on the number of connections that can stay open, and max_overflow is a maximum limit on the number of connections that can be temporarily opened when the pool has been consumed. It looks like the defaults are 5 and 10 which would keep 5 connections open all the time and allow 10 temp.

Do I need to set max_pool_size to 0 and max_overflow to the number of connections that I want to allow? Is that a reasonable and correct configuration? Intuitively that doesn't seem right, to have a pool size of 0, but if the "pool" is a group of connections that will remain open until they time out, then maybe 0 is correct?

From: Albert Braden <Albert.Braden@synopsys.com>
Sent: Wednesday, September 4, 2019 10:19 AM
To: openstack-discuss@lists.openstack.org
Cc: Gaëtan Trellu <gaetan.trellu@incloudus.com>
Subject: RE: Nova causes MySQL timeouts

We’re not setting max_pool_size nor max_overflow option presently. I googled around and found this document:

https://docs.openstack.org/keystone/stein/configuration/config-options.html

Document says:

[api_database]

connection_recycle_time = 3600               (Integer) Timeout before idle SQL connections are reaped.

max_overflow = None                                   (Integer) If set, use this value for max_overflow with SQLAlchemy.

max_pool_size = None                                  (Integer) Maximum number of SQL connections to keep open in a pool.

[database]

connection_recycle_time = 3600               (Integer) Timeout before idle SQL connections are reaped.

min_pool_size = 1                                            (Integer) Minimum number of SQL connections to keep open in a pool.

max_overflow = 50                                          (Integer) If set, use this value for max_overflow with SQLAlchemy.

max_pool_size = None                                  (Integer) Maximum number of SQL connections to keep open in a pool.

If min_pool_size is >0, would that cause at least 1 connection to remain open until it times out? What are the recommended values for these, to allow unused connections to close before they time out? Is “min_pool_size = 0” an acceptable setting?

My settings are default:

[api_database]:

#connection_recycle_time = 3600

#max_overflow = <None>

#max_pool_size = <None>

[database]:

#connection_recycle_time = 3600

#min_pool_size = 1

#max_overflow = 50

#max_pool_size = 5

It’s not obvious what max_overflow does. Where can I find a document that explains more about these settings?

From: Gaëtan Trellu <gaetan.trellu@incloudus.com>
Sent: Tuesday, September 3, 2019 1:37 PM
To: Albert Braden <albertb@synopsys.com>
Cc: openstack-discuss@lists.openstack.org
Subject: Re: Nova causes MySQL timeouts

Hi Albert,

It is a configuration issue, have a look to max_pool_size and max_overflow options under [database] section. 

Keep in mind than more workers you will have more connections will be opened on the database.

Gaetan (goldyfruit)

On Sep 3, 2019 4:31 PM, Albert Braden <Albert.Braden@synopsys.com> wrote:

It looks like nova is keeping mysql connections open until they time out. How are others responding to this issue? Do you just ignore the mysql errors, or is it possible to change configuration so that nova closes and reopens connections before they time out? Or is there a way to stop mysql from logging these aborted connections without hiding real issues?

Aborted connection 10726 to db: 'nova' user: 'nova' host: 'asdf' (Got timeout reading communication packets)