On 2018-05-01 13:43:14 -0400, Robert Haas wrote: > On Tue, May 1, 2018 at 1:41 PM, Andres Freund <and...@anarazel.de> wrote: > > I unfortunately don't have access to the relevant reports anymore, so > > it's only by memory. What I do remember is that a few I saw > > pg_stat_bgwriter.buffers_backend_fsync values that we a pretty sizable > > fraction of the buffers written by backends. I don't think I ever > > figured out how problematic that was from a peformance perspective, and > > how large a fraction of the overall number of fsyncs those were. > > > > One was a workload with citus (lots of tables per node), and one was > > inheritance based partitioning. There were a few others too, where I > > don't recall anything about the workload. > > Hmm. Partitioning probably does make it easier to overrun the queue, > but even so it seems hard -- the queue has one entry per shared > buffer, which is a lot.
Yea, I really don't remember the details unfortunately. I guess if you have a large number of tables and then a large number of corresponding relations (indexes, sequences) and there's some temporal locality of which tables are accessed, it's not insane to think you could exceed NBuffers relations. As I said, I'm not sure whether this caused actual performance issues, just that I saw the higher value (there were enough architectual issues to fix...). Greetings, Andres Freund
