On Thu, Jan 5, 2017 at 12:33 AM, Robert Haas <robertmh...@gmail.com> wrote: > On Wed, Jan 4, 2017 at 9:47 AM, Simon Riggs <si...@2ndquadrant.com> wrote: >> On 4 January 2017 at 13:57, Robert Haas <robertmh...@gmail.com> wrote: >>> On Wed, Jan 4, 2017 at 3:05 AM, Simon Riggs <si...@2ndquadrant.com> wrote: >>>> Strange response. Nothing has been assumed. I asked for tests and you >>>> provided measurements. >>> >>> Sure, of zero-filling a file with dd. But I also pointed out that in >>> a real PostgreSQL cluster, the change could actually *reduce* latency. >> >> I think we are talking at cross purposes. We agree that the main >> change is useful, but it causes another problem which I can't see how >> you can characterize as reduced latency, based upon your own >> measurements. > > Zero-filling files will take longer if the files are bigger. That > will increase latency. But we will also have fewer forced > end-of-segment syncs. That will reduce latency. Which effect is > bigger?
It depends on if the environment is CPU-bounded or I/O bounded. If CPU is at its limit, zero-filling takes time. If that's the I/O, fsync() would take longer to complete. -- Michael -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers