Greg Smith escribió: > On 7/18/13 11:04 AM, Robert Haas wrote: > >On a system where fsync is sometimes very very slow, that > >might result in the checkpoint overrunning its time budget - but SO > >WHAT? > > Checkpoints provide a boundary on recovery time. That is their only > purpose. You can always do better by postponing them, but you've > now changed the agreement with the user about how long recovery > might take. > > And if you don't respect the checkpoint boundary, what you can't do > is then claim better execution performance than something that did. > It's always possible to improvement throughput by postponing I/O. > SO WHAT? If that's OK, you don't need complicated logic to do that. > Increase checkpoint_timeout. The system with checkpoint_timeout at > 6 minutes will always outperform one where it's 5.
I think the idea is to have a system in which most of the time the recovery time will be that for checkpoint_timeout=5, but in those (hopefully rare) cases where checkpoints take a bit longer, the recovery time will be that for checkpoint_timeout=6. In any case, if the system crashes past minute 5 after the previous checkpoint (the worst possible timing), the current checkpoint will have already started, so recovery will take slightly less time because some flush work had already been done. -- Álvaro Herrera http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers