I would also love to see these changes applied. With regards to the bugs around not issuing a commit or rollback, is it possible to have sqlachemy track whether or not a transaction starts and only issue a rollback when a session is handed back with an open transaction on it? Seems like a useful defensive measure.
Mark On Wed, Jul 11, 2012 at 12:46 PM, Jay Pipes <[email protected]> wrote: > +1 to all your ideas below, Devananda. > > On 07/11/2012 01:33 PM, Devananda van der Veen wrote: > > Hi all, > > > > I've been taking a look at the way Nova uses its MySQL database. Having > > done MySQL performance audits for years as a consultant, a few things > > jumped out right away at me. First is the way that SQLAlchemy is > > wrapping nearly every query in an unnecessary "ping check" and rollback, > > eg.: > > > > select 1; > > select ... from ... where ...; > > rollback; > > select 1; > > update ... where ...; > > commit; > > rollback; > > > > > > You can find a complete sample here: > http://paste.openstack.org/show/18731/ > > > > I think I understand the reason for both the "select 1" and the > > "rollback" statements. However, in the interest of performance for > > large-scale deployments, I feel pretty strongly that they need to go > away. > > > > As I see it, there are three factors here. > > > > (I) Most of the code in db/sqlalchemy/api.py abstracts a "unit of work" > > to a very low level, generally a single database read or write, and does > > not currently support transactions spanning multiple consistent writes. > > This is why "select 1" and "rollback" appear around almost every query > > -- most functions in api.py is checking out a session object, doing one > > or two queries, and then releasing the session. This actually creates a > > much larger issue about transactional atomicity for larger operations, > > such as the open bug about network creation here, and is probably better > > for another discussion. > > https://bugs.launchpad.net/nova/+bug/755138. > > > > (II) Connections are tested by the MySQLPingListener function, using > > "SELECT 1" statements, every time a connection / session is checked out > > of the pool. Since connections are usually OK, this adds overhead > > unnecessarily. It would be more efficient to handle the errors when > > connections aren't OK. I've opened a bug with a description of one > > possible way to fix this issue. There are probably other viable > > solutions as well. > > https://bugs.launchpad.net/nova/+bug/1007027 > > > > My understanding is that this was implemented to prevent "Database has > > gone away" errors from occurring every time that the database is > > restarted, or when connections are closed by the database after being > > idle for long periods. In my opinion, a better solution to these > > problems is to: > > * wrap queries in retry logic, which will catch disconnect errors and > > attempt to reconnect to the database. I have a patch for this, if folks > > are interested. > > * set Nova's sql_idle_timeout to less than MySQL's wait_timeout, and set > > them both to a reasonably short interval (eg, 1 minute), rather than the > > default (which I think is 8 hours). > > > > (III) Transaction state is reset when connections are returned to the > > pool, even if no transaction was in progress, or the > > transaction-in-progress already committed. This is completely wasteful, > > and easy to disable. > > https://bugs.launchpad.net/nova/+bug/1007038 > > > > Caveat here is that this reset-on-return functionality *is* useful when > > code doesn't properly clean up its own transaction state. When I turned > > it off, it exposed a few bugs, which I haven't tracked down yet. > > Lowering the sql_idle_timeout will provide the same "solution" as the > > current reset-on-return behavior in that it will prevent long-running > > idle transactions that tie up resources unnecessarily. > > > > > > In summary, I'd like to see Nova stop spamming the database with "SELECT > > 1" and "ROLLBACK", and think this should be pretty easy to do. Testing > > these two changes in my devstack seems to work fine, but I'd like to > > know what others think before I propose a changeset with this kind of > > potential impact. > > > > Cheers, > > Devananda > > > > > > _______________________________________________ > > Mailing list: https://launchpad.net/~openstack > > Post to : [email protected] > > Unsubscribe : https://launchpad.net/~openstack > > More help : https://help.launchpad.net/ListHelp > > > > > _______________________________________________ > Mailing list: https://launchpad.net/~openstack > Post to : [email protected] > Unsubscribe : https://launchpad.net/~openstack > More help : https://help.launchpad.net/ListHelp >
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