On 9/26/09 8:19 AM, Greg Smith wrote:
> This means that the question you want an answer to is "if the OS cache
> isn't really available, where does giving memory to shared_buffers
> becomes less efficient than not caching things at all?" My guess is
> that this number is much larger than 10GB, but I don't think anyone has
> done any tests to try to quantify exactly where it is. Typically when
> people are talking about systems as large as yours, they're dedicated
> database servers at that point, so the OS cache gets considered at the
> same time. If it's effectively out of the picture, the spot where
> caching still helps even when it's somewhat inefficient due to buffer
> contention isn't well explored.
It also depends on the filesystem. In testing at Sun and on this list,
people have found that very large s_b (60% of RAM) plus directIO was
actually a win on Solaris UFS, partly because UFS isn't very agressive
or smart about readahead and caching. On Linux/Ext3, however, it was
never a win.
I don't know what AIX's filesystems are like.
--
Josh Berkus
PostgreSQL Experts Inc.
www.pgexperts.com