Hi, A page may be double buffered in PG's buffer pool and in OS's buffer cache. Other DBMS like DB2 and Oracle has provided Direct I/O option to eliminate double buffering. I noticed there were discusses on the list. But I can not find similar option in PG. Does PG support direct I/O now? The tuning guide of PG usually recommends a small shared buffer pool (compared to the size of physical memory). I think it is to avoid swapping. If there were swapping, OS kernel may swap out some pages in PG's buffer pool even PG want to keep them in memory. i.e. PG would loose full control over buffer pool. A large buffer pool is not good because it may 1. cause more pages double buffered, and thus decrease the efficiency of buffer cache and buffer pool. 2. may cause swapping. Am I right? If PG's buffer pool is small compared with physical memory, can I say that the hit ratio of PG's buffer pool is not so meaningful because most misses can be satisfied by OS Kernel's buffer cache? Thanks! Xiaoning
