Re: VACUUM always makes all pages dirty

ITAGAKI Takahiro wrote:
> VACUUM in 8.3dev always makes all pages dirty even if there are no jobs.
> In 8.2.5, VACUUM produces no dirty pages in the same workload. Therefore,
> VACUUM on 8.3 takes longer time than 8.2. I doubt some bugs in the
> HOT-related codes here, but I cannot point out the actual position yet...

Yeah, it's definitely a HOT-introdued thing. Vacuum calls
heap_page_prune on every page, and this in heap_page_prune is dirtying
the buffer:

> else
> {
> /*
> * If we didn't prune anything, we have nonetheless updated the
> * pd_prune_xid field; treat this as a non-WAL-logged hint.
> */
> SetBufferCommitInfoNeedsSave(buffer);
> }

I don't have time to dig deeper at this moment. I'll take a look later
today, unless someone beats me to it. We obviously don't want to call
SetBufferCommitInfoNeedsSave if we didn't really change the pd_prune_xid
field.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com