On Fri, 15 Jun 2007, Gregory Stark wrote:
If I understand it right Greg Smith's concern is that in a busier system where even *with* the load distributed checkpoint the i/o bandwidth demand during t he checkpoint was *still* being pushed over 100% then spreading out the load would only exacerbate the problem by extending the outage.
Thank you for that very concise summary; that's exactly what I've run into. DBT2 creates a heavy write load, but it's not testing a real burst behavior where something is writing as fast as it's possible to.
I've been involved in applications that are more like a data logging situation, where periodically you get some data source tossing transactions in as fast as it will hit disk--the upstream source temporarily becomes faster at generating data during these periods than the database itself can be. Under normal conditions, the LDC smoothing would be a win, as it would lower the number of times the entire flow of operations got stuck. But at these peaks it will, as you say, extend the outage.
It might even make sense to run a test with an outright overloaded to see if the patch doesn't exacerbate the condition.
Exactly. I expect that it will make things worse, but I'd like to keep an eye on making sure the knobs are available so that it's only slightly worse.
I think it's important to at least recognize that someone who wants LDC normally might occasionally have a period where they're completely overloaded, and that this new feature doesn't have an unexpected breakdown when that happens. I'm still stuggling with creating a simple test case to demonstrate what I'm concerned about. I'm not familiar enough with the TPC testing to say whether your suggestions for adjusting warehouse size would accomplish that (because the flow is so different I had to abandon working with that a while ago as not being representative of what I was doing), but I'm glad you're thinking about it.
-- * Greg Smith [EMAIL PROTECTED] http://www.gregsmith.com Baltimore, MD ---------------------------(end of broadcast)--------------------------- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
