Re: Request for further clarification on synchronous_commit

On Tue, Aug 18, 2020 at 10:58:51AM -0400, Bruce Momjian wrote:
> Un, 'on' does _not_ apply the WAL data, and remote_apply does do remote
> fsync. If you want to go in order of severity, with the most severe
> first, it is:
>
> remote_apply
> on
> remote_write
> local
>
> This is seen in the C enum ordering for synchronous_commit, but in
> reverse order:
>
> typedef enum
> {
> SYNCHRONOUS_COMMIT_OFF, /* asynchronous commit */
> SYNCHRONOUS_COMMIT_LOCAL_FLUSH, /* wait for local flush only */
> SYNCHRONOUS_COMMIT_REMOTE_WRITE, /* wait for local flush and remote
> * write */
> SYNCHRONOUS_COMMIT_REMOTE_FLUSH, /* wait for local and remote flush */
> SYNCHRONOUS_COMMIT_REMOTE_APPLY /* wait for local flush and remote apply */
> } SyncCommitLevel;

Also, there is some logic to say that the postgresql.conf
synchronous_commit options list should be reordered from:

#synchronous_commit = on # synchronization level;
# off, local, remote_write, remote_apply, or on

to

#synchronous_commit = on # synchronization level;
# off, local, remote_write, on, or remote_apply

I think we should backpatch the doc changes, but maybe not the
postgresql.conf one --- I am not sure.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EnterpriseDB https://enterprisedb.com

The usefulness of a cup is in its emptiness, Bruce Lee