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List:       postgresql-general
Subject:    Re: [HACKERS] Is pg_control file crashsafe?
From:       Bruce Momjian <bruce () momjian ! us>
Date:       2016-04-30 21:55:08
Message-ID: 20160430215508.GC556 () momjian ! us
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On Thu, Apr 28, 2016 at 09:58:00PM +0000, Alex Ignatov wrote:
> Hello everyone!
> We have some issue with truncated pg_control file on Windows after power
> failure.
> My questions is : 
> 1) Is pg_control protected from say , power crash or partial write? 
> 2) How PG update pg_control? By writing in it or writing in some temp file and
> after that rename it to pg_control to be atomic?

We write pg_controldata in one write() OS call:

    if (write(fd, buffer, PG_CONTROL_SIZE) != PG_CONTROL_SIZE)

> 3) Can PG have  multiple pg_control copy to be more fault tolerant?
> 
> PS During some experiments we found that at present time there is no any method
> to do crash recovery with "restored" version of pg_control (based on some
> manipulations with pg_resetxlog ).
>  Only by using pg_resetxlog and setting it parameters to values taken from wal
> file (pg_xlogdump)we can at least start PG and saw that PG state is at the
> moment of last check point. But we have no real confidence that PG is in
> consistent state(also docs on pg_resetxlogs told us about it too)

We have talked about improving the reliability of pg_control, but
failures are so rare we have never done anything to improve it.  I know
Tatsuo has talked about making pg_control more reliable, so I am CC'ing
him.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

+ As you are, so once was I. As I am, so you will be. +
+                     Ancient Roman grave inscription +


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