Re: a question about Direct I/O and double buffering

On Thu, 2007-04-05 at 13:09 -0500, 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.