Re: Which process is actually doing the WAL writes/calls XLogFlush?

On 06/23/2017 05:50 AM, Daniel Westermann wrote:
> Hi all,
>
> as I did not find the answer in the documentation: Which background
> process is actually doing the writes/flushes to the WAL? In the docs
> (https://www.postgresql.org/docs/10/static/wal-configuration.html) it is
> explained which internal functions are responsible for this:
> XLogInsertRecord and XLogFlush but who does call them, especially the
> flush? Is it the process for the user connection itself then which calls
> this after commit?

For all the details see here:

https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/access/transam/README;h=4ae4715339e707ab5ed879a628aa96b73002ef43;hb=6a18e4bc2d13d077c52cf90a4c6ec68343808ba7

>
> Initially I thought it is the job of the wal_writer but according to
> this blog post
> (http://www.cybertec.at/postgresql-writer-and-wal-writer-processes-explained/)
> this seems not to be true, right? Why do I need the wal_writer at all
> then when synchronous_commit is set to something else than off?

See here:

https://www.postgresql.org/docs/9.6/static/wal-async-commit.html

>
> Thanks for your help
> Daniel

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com