On 08/10/2016 08:28 AM, Robert Haas wrote:
On Wed, Aug 10, 2016 at 11:09 AM, Heikki Linnakangas <hlinn...@iki.fi> wrote:
Still, having to invent CSNs seems like a huge loss for this design.
Personally I'd give up async commit first.  If we had only sync commit,
the rule could be "xact LSN less than snapshot threshold and less than
WAL flush position", and we'd not need CSNs.  I know some people like
async commit, but it's there because it was easy and cheap in our old
design, not because it's the world's greatest feature and worth giving
up performance for.

I don't think that's a very popular opinion (I disagree, for one).
Asynchronous commits are a huge performance boost for some applications. The
alternative is fsync=off, and I don't want to see more people doing that.
SSDs have made the penalty of an fsync much smaller, but it's still there.

Uh, yeah.  Asynchronous commit can be 100 times faster on some
realistic workloads.  If we remove it, many people will have to decide
between running with fsync=off and abandoning PostgreSQL altogether.
That doesn't strike me as a remotely possible line of attack.

+1 for Robert here, removing async commit is a non-starter. It is PostgreSQL performance 101 that you disable synchronous commit unless you have a specific data/business requirement that needs it. Specifically because of how much faster Pg is with async commit.



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