Subject: Re: Proposal for CSN based snapshots
To: Robert Haas <robertmhaas@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>
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From: "Joshua D. Drake" <jd@commandprompt.com>
Organization: Command Prompt, Inc.
Message-ID: <3406288c-9989-5c95-4445-5443b7a7b66f@commandprompt.com>
Date: Wed, 10 Aug 2016 08:54:48 -0700
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On 08/10/2016 08:28 AM, Robert Haas wrote:
> On Wed, Aug 10, 2016 at 11:09 AM, Heikki Linnakangas <hlinnaka@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.

Sincerely,

jD


>


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