On Thu, Feb 26, 2004 at 06:06:06PM -0500, Christopher Weimann wrote: > On 02/26/2004-11:16AM, Dror Matalon wrote: > > > > > > effective_cache_size changes no cache settings for postgresql, it simply > > > acts as a hint to the planner on about how much of the dataset your OS / > > > Kernel / Disk cache can hold. > > > > I understand that. The question is why have the OS, in this case FreeBsd > > use only 200 Megs for disk cache and not more. Why not double the > > vfs.hibufspace to 418119680 and double the effective_cache_size to 51040. > > > > FreeBSD uses ALL ram that isn't being used for something else as > its disk cache. The "effective_cache_size" in the PostGreSQL config > has no effect on how the OS chooses to use memory, it is just hint > to the PostGreSQL planner so it can guess the the likelyhood of > what it is looking for being in the cache.
Let me try and say it again. I know that setting effective_cache_size doesn't affect the OS' cache. I know it just gives Postgres the *idea* of how much cache the OS is using. I know that. I also know that a correct hint helps performance. I've read Matt Dillon's discussion about the freebsd VM at http://www.daemonnews.org/200001/freebsd_vm.html and I didn't see him saying that Freebsd uses all the free RAM for disk cache. Would you care to provide a URL pointing to that? Assuming you are correct, why has the ongoing recommendation been to use hibufspace/8192 as the effective_cache_size? Seems like it would be quite a bit more on machines with lots of RAM. Regards, Dror -- Dror Matalon Zapatec Inc 1700 MLK Way Berkeley, CA 94709 http://www.fastbuzz.com http://www.zapatec.com ---------------------------(end of broadcast)--------------------------- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faqs/FAQ.html