Re: [HACKERS] WAL logging problem in 9.4.3?

From: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
To: 9erthalion6(at)gmail(dot)com
Cc: andrew(dot)dunstan(at)2ndquadrant(dot)com, hlinnaka(at)iki(dot)fi, robertmhaas(at)gmail(dot)com, michael(at)paquier(dot)xyz, pgsql-hackers(at)postgresql(dot)org
Subject: Re: [HACKERS] WAL logging problem in 9.4.3?
Date: 2018-12-20 08:32:25
Message-ID: 20181220.173225.94657882.horiguchi.kyotaro@lab.ntt.co.jp
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Hello.

At Fri, 30 Nov 2018 18:27:05 +0100, Dmitry Dolgov <9erthalion6(at)gmail(dot)com> wrote in <CA+q6zcV6MUg1BEoQUywX917Oiz6JoMdoZ1Vu3RT5GgBb-yPszg(at)mail(dot)gmail(dot)com>
> > On Wed, Nov 14, 2018 at 4:48 AM Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> >
> > 0004 was shot by e9edc1ba0b. Rebased to the current HEAD.
> > Successfully built and passeed all regression/recovery tests
> > including additional recovery/t/016_wal_optimize.pl.
>
> Thank you for working on this patch. Unfortunately, cfbot complains that
> v4-0004-Fix-WAL-skipping-feature.patch could not be applied without conflicts.
> Could you please post a rebased version one more time?

Thanks. Here's the rebased version. I found no other amendment
required other than the apparent conflict.

> > On Fri, Jul 27, 2018 at 9:26 PM Andrew Dunstan <andrew(dot)dunstan(at)2ndquadrant(dot)com> wrote:
> >
> > On 07/18/2018 10:58 AM, Heikki Linnakangas wrote:
> > > On 18/07/18 16:29, Robert Haas wrote:
> > >> On Wed, Jul 18, 2018 at 9:06 AM, Michael Paquier
> > >> <michael(at)paquier(dot)xyz> wrote:
> > >>>> What's wrong with the approach proposed in
> > >>>> http://postgr.es/m/55AFC302.1060805@iki.fi ?
> > >>>
> > >>> For back-branches that's very invasive so that seems risky to me
> > >>> particularly seeing the low number of complaints on the matter.
> > >>
> > >> Hmm. I think that if you disable the optimization, you're betting that
> > >> people won't mind losing performance in this case in a maintenance
> > >> release. If you back-patch Heikki's approach, you're betting that the
> > >> committed version doesn't have any bugs that are worse than the status
> > >> quo. Personally, I'd rather take the latter bet. Maybe the patch
> > >> isn't all there yet, but that seems like something we can work
> > >> towards. If we just give up and disable the optimization, we won't
> > >> know how many people we ticked off or how badly until after we've done
> > >> it.
> > >
> > > Yeah. I'm not happy about backpatching a big patch like what I
> > > proposed, and Kyotaro developed further. But I think it's the least
> > > bad option we have, the other options discussed seem even worse.
> > >
> > > One way to review the patch is to look at what it changes, when
> > > wal_level is *not* set to minimal, i.e. what risk or overhead does it
> > > pose to users who are not affected by this bug? It seems pretty safe
> > > to me.
> > >
> > > The other aspect is, how confident are we that this actually fixes the
> > > bug, with least impact to users using wal_level='minimal'? I think
> > > it's the best shot we have so far. All the other proposals either
> > > don't fully fix the bug, or hurt performance in some legit cases.
> > >
> > > I'd suggest that we continue based on the patch that Kyotaro posted at
> > > https://www.postgresql.org/message-id/20180330.100646.86008470.horiguchi.kyotaro%40lab.ntt.co.jp.
> > >
> > I have just spent some time reviewing Kyatoro's patch. I'm a bit
> > nervous, too, given the size. But I'm also nervous about leaving things
> > as they are. I suspect the reason we haven't heard more about this is
> > that these days use of "wal_level = minimal" is relatively rare.
>
> I'm totally out of context of this patch, but reading this makes me nervous
> too. Taking into account that the problem now is lack of review, do you have
> plans to spend more time reviewing this patch?

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center

Attachment Content-Type Size
v5-0001-TAP-test-for-copy-truncation-optimization.patch text/x-patch 7.2 KB
v5-0002-Write-WAL-for-empty-nbtree-index-build.patch text/x-patch 2.1 KB
v5-0003-Add-infrastructure-to-WAL-logging-skip-feature.patch text/x-patch 22.6 KB
v5-0004-Fix-WAL-skipping-feature.patch text/x-patch 16.4 KB

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