Re: Estimating seq_page_fetch and random_page_fetch

"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

> "Umar Farooq Minhas" <umarfm13(at)hotmail(dot)com> writes:
>> How can we accrately estimate the "seq_page_fetch" and =
>> "random_page_fetch" costs from outside the postgres using for example a =
>> C routine.
>
> Use a test case larger than memory. Repeat many times to average out
> noise. IIRC, when I did the experiments that led to the current
> random_page_cost of 4.0, it took about a week before I had numbers I
> trusted.

When I was running tests I did it on a filesystem where nothing else was
running. Between tests I unmounted and remounted it. As I understand it Linux
associates the cache with the filesystem and not the block device and discards
all pages from cache when the filesystem is unmounted.

That doesn't contradict anything Tom said, it might be useful as an additional
tool though.

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com