On 4/5/17 7:29 AM, Simon Riggs wrote: > On 5 April 2017 at 06:04, Beena Emerson <memissemer...@gmail.com> wrote: >> >> The WALfilename - LSN mapping disruption for higher values you mean? Is >> there anything else I have missed? > > I see various issues raised but not properly addressed > > 1. we would need to drop support for segment sizes < 16MB unless we > adopt a new incompatible filename format. > I think at 16MB the naming should be the same as now and that > WALfilename -> LSN is very important. > For this release, I think we should just allow >= 16MB and avoid the > issue thru lack of time.
+1. > 2. It's not clear to me the advantage of being able to pick varying > filesizes. I see great disadvantage in having too many options, which > greatly increases the chance of incompatibility, annoyance and > breakage. I favour a small number of values that have been shown by > testing to be sweet spots in performance and usability. (1GB has been > suggested) I'm in favor of 16,64,256,1024. > 3. New file allocation has been a problem raised with this patch for > some months now. I've been playing around with this and I don't think short tests show larger sizes off to advantage. Larger segments will definitely perform more poorly until Postgres starts recycling WAL. Once that happens I think performance differences should be negligible, though of course this needs to be verified with longer-running tests. If archive_timeout is set then there will also be additional time taken to zero out potentially larger unused parts of the segment. I don't see this as an issue, however, because if archive_timeout is being triggered then the system is very likely under lower than usual load. > Lack of clarity and/or movement on these issues is very, very close to > getting the patch rejected now. Let's not approach this with the > viewpoint that I or others want it to be rejected, lets work forwards > and get some solid changes that will improve the world without causing > problems. I would definitely like to see this go in, though I agree with Peter that we need a lot more testing. I'd like to see some build farm animals testing with different sizes at the very least, even if there's no time to add new tests. -- -David da...@pgmasters.net -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers