Re: The shared buffers challenge

On Fri, May 27, 2011 at 7:19 PM, Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
> On Thu, 2011-05-26 at 09:31 -0500, Merlin Moncure wrote:
>> Where they are most helpful is for masking of i/o if
>> a page gets dirtied >1 times before it's written out to the heap
>
> Another possible benefit of higher shared_buffers is that it may reduce
> WAL flushes. A page cannot be evicted from shared_buffers until the WAL
> has been flushed up to the page's LSN ("WAL before data"); so if there
> is memory pressure to evict dirty buffers, it may cause extra WAL
> flushes.
>
> I'm not sure what the practical effects of this are, however, but it
> might be an interesting thing to test.

Hm, I bet it could make a fairly big difference if wal data is not on
a separate volume.

merlin