On Tue, Mar 18, 2025 at 5:04 PM Andres Freund <andres@anarazel.de> wrote:
Is that actually a good description of what we assume? I don't know where that 90% is coming from?
That one's all my fault. It was an attempt to curve-fit backwards why the 4.0 number Tom set with his initial commit worked as well as it did given that underlying storage was closer to 50X as slow, and I sold the idea well enough for Bruce to follow the reasoning and commit it. Back then there was a regular procession of people who measured the actual rate and wondered why there was the order of magnitude difference between those measurements and the parameter. Pointing them toward thinking in terms of the cached read percentage too did a reasonable job of deflecting them onto why the model was more complicated than it seems. I intended to follow that up with more measurements, only to lose the whole project into a non-disclosure void I have only recently escaped
I agree with your observation that the underlying cost of a non-sequential read stall on cloud storage is not markedly better than the original random: sequential ratio of mechanical drives. And the PG17 refactoring to improve I/O chunking worked to magnify that further.
The end of this problem I'm working on again is assembling some useful mix of workloads such that I can try changing one of these magic constants with higher confidence. My main working set so far is write performance regression test sets against the Open Street Map loading workload, that I've been blogging about, plus the old read-only queries of the SELECT-only spaced along a scale/client grid. My experiments so far have been around another Tom special, the maximum buffer usage count limit, which turned into another black hole full of work I have only recently escaped. I haven't really thought much yet about a workload set that would allow adjusting random_page_cost. On the query side we've been pretty heads down on the TPC-H and Clickbench sets. I don't have buffer internals data from those yet though, will have to add that to the work queue.
