On 4/5/07, Xiaoning Ding <dingxn@xxxxxxxxxxxxxxxxxx> wrote:
Alex Deucher wrote: > 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 I am more concerned with reads , and how to do direct I/O under Linux here. Reading raw devices in linux bypasses OS buffer cache. But how can you mount a raw device( it is a character device) as a file system?
In this case, I guess you'd probably have to do it within pg itself. Alex
