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FYI: I've moved the spec to oslo space since the switch is not really
limited to neutron, and most of coding is to be done in oslo.db
(though not much anyway).

New spec: https://review.openstack.org/#/c/108355/

On 09/07/14 13:17, Ihar Hrachyshka wrote:
> Hi all,
> 
> Multiple projects are suffering from db lock timeouts due to
> deadlocks deep in mysqldb library that we use to interact with
> mysql servers. In essence, the problem is due to missing eventlet
> support in mysqldb module, meaning when a db lock is encountered,
> the library does not yield to the next green thread, allowing other
> threads to eventually unlock the grabbed lock, and instead it just
> blocks the main thread, that eventually raises timeout exception
> (OperationalError).
> 
> The failed operation is not retried, leaving failing request not 
> served. In Nova, there is a special retry mechanism for deadlocks, 
> though I think it's more a hack than a proper fix.
> 
> Neutron is one of the projects that suffer from those timeout
> errors a lot. Partly it's due to lack of discipline in how we do
> nested calls in l3_db and ml2_plugin code, but that's not something
> to change in foreseeable future, so we need to find another
> solution that is applicable for Juno. Ideally, the solution should
> be applicable for Icehouse too to allow distributors to resolve
> existing deadlocks without waiting for Juno.
> 
> We've had several discussions and attempts to introduce a solution
> to the problem. Thanks to oslo.db guys, we now have more or less
> clear view on the cause of the failures and how to easily fix them.
> The solution is to switch mysqldb to something eventlet aware. The
> best candidate is probably MySQL Connector module that is an
> official MySQL client for Python and that shows some (preliminary)
> good results in terms of performance.
> 
> I've posted a Neutron spec for the switch to the new client in Juno
> at [1]. Ideally, switch is just a matter of several fixes to
> oslo.db that would enable full support for the new driver already
> supported by SQLAlchemy, plus 'connection' string modified in
> service configuration files, plus documentation updates to refer to
> the new official way to configure services for MySQL. The database
> code won't, ideally, require any major changes, though some
> adaptation for the new client library may be needed. That said,
> Neutron does not seem to require any changes, though it was
> revealed that there are some alembic migration rules in Keystone or
> Glance that need (trivial) modifications.
> 
> You can see how trivial the switch can be achieved for a service
> based on example for Neutron [2].
> 
> While this is a Neutron specific proposal, there is an obvious wish
> to switch to the new library globally throughout all the projects,
> to reduce devops burden, among other things. My vision is that,
> ideally, we switch all projects to the new library in Juno, though
> we still may leave several projects for K in case any issues arise,
> similar to the way projects switched to oslo.messaging during two
> cycles instead of one. Though looking at how easy Neutron can be
> switched to the new library, I wouldn't expect any issues that
> would postpone the switch till K.
> 
> It was mentioned in comments to the spec proposal that there were
> some discussions at the latest summit around possible switch in
> context of Nova that revealed some concerns, though they do not
> seem to be documented anywhere. So if you know anything about it,
> please comment.
> 
> So, we'd like to hear from other projects what's your take on that 
> move, whether you see any issues or have concerns about it.
> 
> Thanks for your comments, /Ihar
> 
> [1]: https://review.openstack.org/#/c/104905/ [2]:
> https://review.openstack.org/#/c/105209/
> 
> _______________________________________________ OpenStack-dev
> mailing list OpenStack-dev at lists.openstack.org 
> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
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