Horton

Re: Refactor pg_rewind code and make it work against a standby

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Andres Freund <andres@anarazel.de>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Soumyadeep Chakraborty <soumyadeep2007@gmail.com>, Kyotaro Horiguchi <horikyota.ntt@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2020-11-20T21:09:34Z
Lists: pgsql-hackers 
On 20/11/2020 19:14, Andres Freund wrote:
> Hi,
> 
> On 2020-11-20 16:19:03 +0200, Heikki Linnakangas wrote:
>> Pushed a fix similar to your patch, but I put the wait_for_catchup() before
>> running pg_rewind. The point of inserting the 'in A, after C was promoted'
>> row is that it's present in B when pg_rewind runs.
> 
> Hm - don't we possibly need *both*? Since post pg_rewind recovery starts
> at the previous checkpoint, it's quite possible for C to get ready to
> answer queries before that record has been replayed?

No, C will not reach consistent state until all the WAL in the source 
system has been replayed. pg_rewind will set minRecoveryPoint to the 
minRecoveryPoint of the source system, after copying all the files. (Or 
its insert point, if it's not a standby server, but in this case it is). 
Same as when taking an online backup.

- Heikki

Commits

  1. Make pg_rewind test case more stable.

  2. Fix timing issue in pg_rewind test.

  3. Allow pg_rewind to use a standby server as the source system.

  4. pg_rewind: Refactor the abstraction to fetch from local/libpq source.

  5. pg_rewind: Replace the hybrid list+array data structure with simplehash.

  6. Refactor pg_rewind for more clear decision making.

  7. pg_rewind: Move syncTargetDirectory() to file_ops.c