On 2015-06-02 17:01:50 +0200, Fabien COELHO wrote: > >The actual problem is sorting & fsyncing in a way that deals efficiently > >with tablespaces, i.e. doesn't write to tablespaces one-by-one. > >Not impossible, but it requires some thought. > > Hmmm... I would have neglected this point in a first approximation, > but I agree that not interleaving tablespaces could indeed loose some > performance.
I think it'll be a hard to diagnose performance regression. So we'll have to fix it. That argument actually was the blocker in previous attempts... > >IMO this feature, if done correctly, should result in better performance > >in 95+% of the workloads > > To demonstrate that would require time... Well, that's part of the contribution process. Obviously you can't test 100% of the problems, but you can work hard with coming up with very adversarial scenarios and evaluate performance for those. > >and be enabled by default. > > I did not had such an ambition with the submitted patch:-) I don't think we want yet another tuning knob that's hard to tune because it's critical for one factor (latency) but bad for another (throughput); especially when completely unnecessarily. > >And that'll not be possible without actually writing mostly sequentially. > > >It's also not just the sequential writes making this important, it's also > >that it allows to do the final fsync() of the individual segments as soon > >as their last buffer has been written out. > > Hmmm... I'm not sure this would have a large impact. The writes are > throttled as much as possible, so fsync will catch plenty other writes > anyway, if there are some. That might be the case in a database with a single small table; i.e. where all the writes go to a single file. But as soon as you have large tables (i.e. many segments) or multiple tables, a significant part of the writes issued independently from checkpointing will be outside the processing of the individual segment. Greetings, Andres Freund -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
