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Re: Proposal for Allow postgresql.conf values to be changed via SQL

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Amit kapila <amit(dot)kapila(at)huawei(dot)com>
Cc: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Dimitri Fontaine <dimitri(at)2ndquadrant(dot)fr>, "cedric(at)2ndquadrant(dot)com" <cedric(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Greg Smith <greg(at)2ndquadrant(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Magnus Hagander <magnus(at)hagander(dot)net>, Christopher Browne <cbbrowne(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
Subject: Re: Proposal for Allow postgresql.conf values to be changed via SQL
Date: 2012-11-24 17:26:02
Message-ID: 12594.1353777962@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Amit kapila <amit(dot)kapila(at)huawei(dot)com> writes:
> On Friday, November 23, 2012 10:10 PM Fujii Masao wrote:
>> What happens if the server crashes while SET PERSISTENT is writing the
>> setting to the file? A partial write occurs and restart of the server would fail
>> because of corrupted postgresql.auto.conf?

> This situation will not happen as SET PERSISTENT command will first write to ".lock" file and then at commit time, 
> rename it to ".auto.conf".

Yes, the right way to write the config file is to write under a
temporary name, fsync the file, and then use rename(2) to atomically
move it into place.  However, the above is contemplating some extra
complexity that I think is useless and undesirable, namely postponing
the rename until commit time.  The point of the suggestion that SET
PERSISTENT not be allowed inside a transaction block is so that you can
write the file immediately rather than have to add commit-time mechanism
to support the feature.  Aside from being extra complexity, and some
extra cycles added in *every single commit*, a post-commit write creates
another way to have post-commit failures, which we cannot cope with in
any sane way.

			regards, tom lane


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