On Fri, 19 Aug 2005 19:12:11 +1000 Graeme Lee <[EMAIL PROTECTED]> wrote:
> It was a suggestion. And the shared buffer cache is still not a disk > cache. A bad suggestion, hence my correcting it. I really hate seeing misinformation in list archives, people do use them. Yes, this is fairly common misinformation about postgresql, even linked to from their site unfortunately, but that doesn't make it true. > http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html > http://www.powerpostgresql.com/PerfList/ Guess what, those are wrong. The first one says: "PLEASE NOTE. PostgreSQL counts a lot on the OS to cache data files and hence does not bother with duplicating its file caching effort." But that would be incredibly stupid, and postgresql's performance would suffer tremendously if it were true. The postgresql source code says that it does use the shared buffer cache to cache pages read from disk, and I believe it over someone who also says things like "The real analysis of the precise best setting is not fully understood" This is absolute nonsense. The shared buffer cache is well understood, and the only way you will hurt performance by making it too big is by using up so much RAM that you start hitting swap, or by making it larger than your data plus the other usage. Unless perhaps your OS performs poorly with large shared memory allocations (does openbsd?). > See above links See src/backend/storage/buffer/bufmgr.c or even just the README file there which refers to "shared disk buffers", and explains about the replacement strategy used. Yes, the shared buffer cache stores queries from each connection until they are executed, that's why the MINIMUM size is max_con * 2. But it also caches pages retrieved from disk, if you gave it more than the bare minimum RAM required just to work at all. The link you sent the first time is correct: "POSTGRESQL does not directly change information on disk. Instead, it requests data be read into the POSTGRESQL shared buffer cache. POSTGRESQL backends then read/write these blocks, and finally flush them back to disk." Or maybe the src/backend/storage/buffer/*.c code is an elaborate hoax and doesn't really do anything? Adam
