On 2013-02-02 18:32:44 +0000, Simon Riggs wrote: > On 2 February 2013 14:24, Andres Freund <and...@2ndquadrant.com> wrote: > > > b) We don't assign the xmin early enough, we only set it when the first > > feedback message arrives, but we should set it when walsender starts > > streaming. > > That's easy to fix.
Not trivial, but not too hard, yes. When the standby initially connects we don't yet know which xid will be required because consistency hasn't yet been achieved. > > c) After a disconnect the feedback message will rather likely ask for an > > xmin horizon thats not valid anymore on the primary. If the disconnect > > was short enough often enough that doesn't matter because nothing has > > been cleared out, but it doesn't really work all that well. > > Thats still better than setting it to the currently valid minimal xmin > > horizon because it prevents cleanup from that moment on. > > I don't see how this can be significantly improved without persistent > > knowledge about standbys. > > We could delay startup of the standby until the xmin on the standby > reaches the xmin on the master. > > So when the standby has hot_standby_feedback = on, at standby > connection we set the xmin of the walsender to be the current value on > the master, then we disallow connections on standby until we have > replayed up to that xmin on the standby. That way the xmin of the > walsender never goes backwards nor do we get cancelations on the > standby. Thats easy enough when the standby is initially started but doesn't work that well if just the connection between both failed (or the master was restarted) and a reconnect worked. To make it work similarly in that case we would have to throw everyone out which would kind of counteract the whole idea. Greetings, Andres Freund -- Andres Freund 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
