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

On 4/5/07, Erik Jones <erik(at)myemma(dot)com> 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