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

From: Alvaro Herrera <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: 2021-07-30 23:51:39
Message-ID: 202107302351.zovw4tdoe47e@alvherre.pgsql
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On 2021-Jul-30, Bossart, Nathan wrote:

> Yes, that was what I was worried about. However, I just performed a
> variety of tests with just 0001 applied, and I am beginning to suspect
> my concerns were unfounded. With wal_buffers set very high,
> synchronous_commit set to off, and a long sleep at the end of
> XLogWrite(), I can reliably cause the archive status files to lag far
> behind the current open WAL segment. However, even if I crash at this
> time, the .ready files are created when the server restarts (albeit
> out of order).

I think that creating files out of order might be problematic. On the
archiver side, pgarch_readyXlog() expects to return the oldest
archivable file; but if we create a newer segment's .ready file first,
it is possible that a directory scan would return that newer file before
the older segment's .ready file appears.

However, the comments in pgarch_readyXlog() aren't super convincing that
processing the files in order is actually a correctness requirement, so
perhaps it doesn't matter all that much.

I noticed that XLogCtl->lastNotifiedSeg is protected by both the
info_lck and ArchNotifyLock. I think it it's going to be protected by
the lwlock, then we should drop the use of the spinlock.

We set archiver's latch on each XLogArchiveNotify(), but if we're doing
it in a loop such as in NotifySegmentsReadyForArchive() perhaps it is
better to create all the .ready files first and do PgArchWakeup() at the
end. I'm not convinced that this is useful but let's at least discard
the idea explicitly if not.

Álvaro Herrera Valdivia, Chile —
"Always assume the user will do much worse than the stupidest thing
you can imagine." (Julien PUYDT)

Attachment Content-Type Size
v2-0001-Avoid-creating-archive-status-.ready-files-too-ea.patch text/x-diff 18.3 KB

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