Re: failover vs. read only queries - Mailing list pgsql-hackers

From Bruce Momjian
Subject Re: failover vs. read only queries
Date
Msg-id 201007020314.o623EHX23635@momjian.us
Whole thread Raw
In response to Re: failover vs. read only queries  (Fujii Masao <masao.fujii@gmail.com>)
Responses Re: failover vs. read only queries
List pgsql-hackers
Fujii Masao wrote:
> On Thu, Jun 10, 2010 at 5:06 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > Josh Berkus <josh@agliodbs.com> writes:
> >> The fact that failover current does *not* terminate existing queries and
> >> transactions was regarded as a feature by the audience, rather than a
> >> bug, when I did demos of HS/SR. ?Of course, they might not have been
> >> thinking of the delay for writes.
> >
> >> If there were an easy way to make the trigger file cancel all running
> >> queries, apply remaining logs and come up, then I'd vote for that for
> >> 9.0. ?I think it's the more desired behavior by most users. ?However,
> >> I'm opposed to any complex solutions which might delay 9.0 release.
> >
> > My feeling about it is that if you want fast failover you should not
> > have your failover target server configured as hot standby at all, let
> > alone hot standby with a long max_standby_delay. ?Such a slave could be
> > very far behind on applying WAL when the crunch comes, and no amount of
> > query killing will save you from that. ?Put your long-running standby
> > queries on a different slave instead.
> >
> > We should consider whether we can improve the situation in 9.1, but it
> > is not a must-fix for 9.0; especially when the correct behavior isn't
> > immediately obvious.
> 
> OK. Let's revisit in 9.1.
> 
> I attached the proposal patch for 9.1. The patch treats max_standby_delay
> as zero (i.e., cancels all the conflicting queries immediately), ever since
> the trigger file is created. So we can cause a recovery to end without
> waiting for any lock held by queries, and minimize the failover time.
> OTOH, queries which don't conflict with a recovery survive the failover.

Should this be added to the first 9.1 commitfest?

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + None of us is going to be here forever. +


pgsql-hackers by date:

Previous
From: Bruce Momjian
Date:
Subject: Re: 9.0beta2 - server crash when using HS + SR
Next
From: Tom Lane
Date:
Subject: Re: failover vs. read only queries