Re: archive status ".ready" files may be created too early

From: "alvherre(at)alvh(dot)no-ip(dot)org" <alvherre(at)alvh(dot)no-ip(dot)org>
To: "Bossart, Nathan" <bossartn(at)amazon(dot)com>
Cc: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>, "x4mmm(at)yandex-team(dot)ru" <x4mmm(at)yandex-team(dot)ru>, "a(dot)lubennikova(at)postgrespro(dot)ru" <a(dot)lubennikova(at)postgrespro(dot)ru>, "hlinnaka(at)iki(dot)fi" <hlinnaka(at)iki(dot)fi>, "matsumura(dot)ryo(at)fujitsu(dot)com" <matsumura(dot)ryo(at)fujitsu(dot)com>, "masao(dot)fujii(at)gmail(dot)com" <masao(dot)fujii(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: archive status ".ready" files may be created too early
Date: 2021-08-18 23:46:44
Message-ID: 202108182346.zkaxdlu27ry4@alvherre.pgsql
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On 2021-Aug-18, Bossart, Nathan wrote:

> As long as XLogBackgroundFlush() found work to do, it would take care
> of notifying, but I don't think we can depend on that. However, since
> we're primarily using the WAL writer to take care of the case when the
> record has already been flushed, notifying beforehand seems fine to
> me. If XLogBackgroundFlush() does end up calling XLogWrite(), it'll
> call it again, anyway.


> In the attached patch, I modified XLogInsertRecord() to simply set the
> latch if we detect that flushRecPtr has advanced.

Right, that's what I was thinking. I modified that slightly to use
LogwrtResult.Flush (which should be fresh enough) instead of calling
GetFlushRecPtr again, which saves a bit. I also changed it to > instead
of >=, because if I understand you correctly we only care to notify if
the flush pointer advanced, not in the case it stayed the same.

I made a few other cosmetic tweaks -- added comment to
SegmentBoundaryEntry and renamed the 'pos' to 'endpos'; renamed argument
'notify' of XLogArchiveNotify to 'nudge' (because having two different
"notify" concepts in that function seemed confusing).

Álvaro Herrera Valdivia, Chile —
"It takes less than 2 seconds to get to 78% complete; that's a good sign.
A few seconds later it's at 90%, but it seems to have stuck there. Did
somebody make percentages logarithmic while I wasn't looking?"

Attachment Content-Type Size
v10-0001-Avoid-creating-archive-status-.ready-files-too-e.patch text/x-diff 21.3 KB

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