On 4/5/07, Erik Jones <erik@xxxxxxxxxx> wrote:
On Apr 5, 2007, at 1:22 PM, Xiaoning Ding wrote: Erik Jones wrote: On Apr 5, 2007, at 12:09 PM, Xiaoning Ding wrote: 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! To the best of my knowledge, Postgres itself does not have a direct IO option (although it would be a good addition). So, in order to use direct IO with postgres you'll need to consult your filesystem docs for how to set the forcedirectio mount option. I believe it can be set dynamically, but if you want it to be permanent you'll to add it to your fstab/vfstab file. I use Linux. It supports direct I/O on a per-file basis only. To bypass OS buffer cache, files should be opened with O_DIRECT option. I afraid that I have to modify PG. Xiaoning Looks like it. I just did a cursory search of the archives and it seems that others have looked at this before so you'll probably want to start there if your up to it.
Linux used to have (still does?) a RAW interface which might also be useful. I think the original code was contributed by oracle so they could support direct IO. Alex
