On Wed, Feb 20, 2013 at 3:44 PM, Josh Berkus <josh@agliodbs.com> wrote:
> On 02/19/2013 07:15 PM, Scott Marlowe wrote:
>> On Tue, Feb 19, 2013 at 4:24 PM, Josh Berkus <josh@agliodbs.com> wrote:
>>> ... then you'll see checkpoint "stalls" and spread checkpoint will
>>> actually make them worse by making the stalls longer.
>>
>> Wait, if they're spread enough then there won't be a checkpoint, so to
>> speak. Are you saying that spreading them out means that they still
>> kind of pile up, even with say a completion target of 1.0 etc?
>
> I'm saying that spreading them makes things worse, because they get
> intermixed with the fsyncs for the WAL and causes commits to stall. I
> tried setting checkpoint_completion_target = 0.0 and throughput got
> about 10% better.
Sounds to me like your IO system is stalling on fsyncs or something
like that. On machines with plenty of IO cranking up completion
target usuall smooths things out. I've got some new big servers
coming in at work over the next few months so I'm gonna test and
compare Ubuntu 10.04 and 12.04 and see if I can see this behaviour.
We have a 12.04 machine in production but honestly it's not working
very hard right now. But it's in production so I can't benchmark it
without causing problems.