Re: XLOG_BLCKSZ vs. wal_buffers table

Simon Riggs wrote:
> On Wed, 2006-05-10 at 09:55 -0700, Mark Wong wrote:
>> Simon Riggs wrote:
>>> Could you turn full_page_writes = off and do a few more tests? I think
>>> the full page writes is swamping the xlog and masking the performance we
>>> might see for normal small xlog writes.
>>> I'd try XLOG_BLCKSZ = 4096 and 8192 to start with. Thanks.
>> Ok, got data for XLOG_BLCKXZ at 4096, 8192, and 32768 with
>> full_page_wirtes = off.  The new data is at the bottom of the page:
>>     http://developer.osdl.org/markw/pgsql/xlog_blcksz.html
> 
> Mark,
> 
> What do you think is causing the step changes at 30+ and 60+ minutes on
> these tests?
> 

I'm at a loss to explain this myself.  The increase in the i/o wait and 
decrease in the response time charts looks backwards to me.  And the 
vmstat charts show that the i/o seems fairly consistent other than 
spikes in i/o chart.  It sort of looks like the operating system stopped 
doing something but I'm not sure how to isolate that any further.

Anyone else have a theory?

Mark