Horton

Re: [Proposal] Fully WAL logged CREATE DATABASE - No Checkpoints

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Dilip Kumar <dilipbalaut@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2021-09-03T21:54:33Z
Lists: pgsql-hackers 
Hi,

On 2021-09-03 14:25:10 +0530, Dilip Kumar wrote:
> Yeah, we can surely lock the relation as described by Robert, but IMHO,
> while creating the database we are already holding the exclusive lock on
> the database and there is no one else allowed to be connected to the
> database, so do we actually need to bother about the lock for the
> correctness?

The problem is that checkpointer, bgwriter, buffer reclaim don't care about
the database of the buffer they're working on... The exclusive lock on the
database doesn't change anything about that. Perhaps you can justify it's safe
because there can't be any dirty buffers or such though.


> I think we already have such a code in multiple places where we bypass the
> shared buffers for copying the relation
> e.g. index_copy_data(), heapam_relation_copy_data().

That's not at all comparable. We hold an exclusive lock on the relation at
that point, and we don't have a separate implementation of reading tuples from
the table or something like that.

Greetings,

Andres Freund

Commits

  1. When using the WAL-logged CREATE DATABASE strategy, bulk extend.

  2. Avoid using a fake relcache entry to own an SmgrRelation.

  3. Fix data-corruption hazard in WAL-logged CREATE DATABASE.

  4. initdb: When running CREATE DATABASE, use STRATEGY = WAL_COPY.

  5. Simplify a needlessly-complicated regular expression.

  6. In 020_createdb.pl, change order of command-line arguments.

  7. Add new block-by-block strategy for CREATE DATABASE.

  8. Fix replay of create database records on standby

  9. Refactor code for reading and writing relation map files.

  10. Replace RelationOpenSmgr() with RelationGetSmgr().

  11. Refactor the fsync queue for wider use.