On 10 May 2013 23:41, Jeff Davis <pg...@j-davis.com> wrote: > On Fri, 2013-05-10 at 18:32 +0100, Simon Riggs wrote: >> We don't write() WAL except with an immediate sync(), so the chances >> of what you say happening are very low to impossible. > > Are you sure? An XLogwrtRqst contains a write and a flush pointer, so I > assume they can be different.
I'm answering this just to complete the discussion. Yes, the write and flush pointer can be different. The write/flush are two actions; we do one first, then the other, very quickly, inside XLogWrite(). But we cannot rely on that, since there are some times when we don't do that, such as wal_buffer overflow and when background walwriter writes are at the end of the ring buffer. Not common, but they do exist and when they exist they write many blocks. Which is counter to what I had said earlier. This part of the proposal isn't necessary for us to get a good answer 95% of the time, so it is dropped. As mentioned on other post, we can write UpdateMinRecoveryPoint(LogwrtResult.Flush, false) every time we do XLogBackgroundFlush() and some other rework to make that happen correctly. I'll post a separate patch. -- Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
