On Tue, Apr 26, 2011 at 7:30 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Apr 14, 2011, at 2:49 AM, Claudio Freire <klaussfreire@gmail.com> wrote:
>> This particular factor is not about an abstract and opaque "Workload"
>> the server can't know about. It's about cache hit rate, and the server
>> can indeed measure that.
>
> The server can and does measure hit rates for the PG buffer pool, but to my knowledge there is no clear-cut way for
PGto know whether read() is satisfied from the OS cache or a drive cache or the platter.
Isn't latency an indicator?
If you plot latencies, you should see three markedly obvious clusters:
OS cache (microseconds), Drive cache (slightly slower), platter
(tail).
I think I had seen a study of sorts somewhere[0]...
Ok, that link is about sequential/random access, but I distinctively
remember one about caches and CAV...
[0] http://blogs.sun.com/brendan/entry/heat_map_analytics
