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

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.

erik jones <erik(at)myemma(dot)com>
software developer
615-296-0838
emma(r)