Re: WAL logging problem in 9.4.3?

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
Cc: michael(dot)paquier(at)gmail(dot)com, david(at)pgmasters(dot)net, hlinnaka(at)iki(dot)fi, simon(at)2ndquadrant(dot)com, tgl(at)sss(dot)pgh(dot)pa(dot)us, andres(at)anarazel(dot)de, masao(dot)fujii(at)gmail(dot)com, kleptog(at)svana(dot)org, pgsql-hackers(at)postgresql(dot)org
Subject: Re: WAL logging problem in 9.4.3?
Date: 2017-04-07 23:33:21
Message-ID: 20170407233321.lpruyvn6r4tuujmt@alvherre.pgsql
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Kyotaro HORIGUCHI wrote:

> The attached patch is quiiiccck-and-dirty-hack of Michael's patch
> just as a PoC of my proposal quoted above. This also passes the
> 006 test. The major changes are the following.
>
> - Moved sync_above and truncted_to into RelationData.

Interesting. I wonder if it's possible that a relcache invalidation
would cause these values to get lost for some reason, because that would
be dangerous.

I suppose the rationale is that this shouldn't happen because any
operation that does things this way must hold an exclusive lock on the
relation. But that doesn't guarantee that the relcache entry is
completely stable, does it? If we can get proof of that, then this
technique should be safe, I think.

In your version of the patch, which I spent some time skimming, I am
missing comments on various functions. I added some as I went along,
including one XXX indicating it must be filled.

RecordPendingSync() should really live in relcache.c (and probably get a
different name).

> X I feel that I have dropped one of the features of the origitnal
> patch during the hack, but I don't recall it clearly now:(

Hah :-)

> X I haven't consider relfilenode replacement, which didn't matter
> for the original patch. (but there's few places to consider).

Hmm ... Please provide.

--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Attachment Content-Type Size
fix-wal-level-minimal-michael-horiguchi-2.patch text/plain 32.3 KB

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