Open Stack

Excerpts from Matt Riedemann's message of 2015-04-16 15:40:30 -0700:
> 
> On 4/16/2015 12:27 PM, Jay Pipes wrote:
> > On 04/16/2015 09:54 AM, Sean Dague wrote:
> >> On 04/16/2015 05:20 PM, Qiming Teng wrote:
> >>>
> >>> Wondering if there is something misconfigured in my devstack
> >>> environment, which was reinstalled on RHEL7 about 10 days ago.
> >>> I'm often running into mysql connections problem as shown below:
> >>>
> >>> $ mysql
> >>> ERROR 1040 (HY000): Too many connections
> >>>
> >>> When I try dump the mysql connection list, I'm getting the followng
> >>> result after a 'systemctl restart mariadb.service':
> >>>
> >>> $ mysqladmin processlist | grep nova | wc -l
> >>> 125
> >>>
> >>> Most of the connections are at Sleep status:
> >>>
> >>> $ mysqladmin processlist | grep nova | grep Sleep | wc -l
> >>> 123
> >>>
> >>> As for the workload, I'm currently only running two VMs in a multi-host
> >>> devstack environment.
> >>>
> >>> So, my questions:
> >>>
> >>>    - Why do we have so many mysql connections from nova?
> >>>    - Is it possible this is caused some misconfigurations?
> >>>    - 125 connections in such a toy setup is insane, any hints on nailing
> >>>      down the connections to the responsible nova components?
> >>>
> >>> Thanks.
> >>>
> >>> Regards,
> >>>    Qiming
> >>
> >> No, that's about right. It's 1 connection per worker. By default most
> >> daemons start 1 worker per processor. Each OpenStack service has a bunch
> >> of daemons. It all adds up pretty quick.
> >
> > And just to add to what Sean says above, there's nothing inherently
> > wrong with sleeping connections to MySQL. What *is* wrong, however, is
> > that the default max_connections setting in my.cnf is 150. :( I
> > frequently recommend upping that to 2000 or more on any modern hardware
> > or decent sized VM.
> >
> > Best,
> > -jay
> >
> > __________________________________________________________________________
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> >
> 
> What do you consider a decent sized VM?  In devstack we default 
> max_connections for postgresql to 200 because we were having connection 
> timeout failures in the gate for pg back in the day:
> 
> http://git.openstack.org/cgit/openstack-dev/devstack/tree/lib/databases/postgresql#n15
> 
> But we don't change this for mysql:
> 
> http://git.openstack.org/cgit/openstack-dev/devstack/tree/lib/databases/mysql
> 
> I think the VMs in the gate are running 8 VCPU + 8 GB RAM, not sure 
> about disk.
> 

Note that pgsql's connection model is vastly different than mysql's. For
mysql, each connection doing nothing uses very little RAM. For pgsql,
IIRC, each one is a forked process.