[CentOS] Centos/Linux Disk Caching, might be OT in some ways

Noob Centos Admin wrote:
> Hi,
> 
>> If you want a fast database forget about file system caching,
>> use Direct I/O and put your memory to better use - application
>> level caching.
> 
> The web application is written in PHP and runs off MySQL and/or
> Postgresql. So I don't think I can access the raw disk data directly,
> nor do I think it would be safe since that bypasses the DBMS's checks.

Which is it? mysql or postgresql or both? Have you actually determined 
that i/o is in fact the bottleneck?

Is the webapp maintaining persistent connections to mysql or is it 
continually connecting and disconnecting? mysql sucks big time if you 
have a good few hundred connections being setup and torn down all the time.

Are complicated relationships being stored in postgresql and not in 
mysql? I do not know how things are now but mysql has a history of only 
being good for simple selects.