Re: Dead Space Map version 2

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

> Vacuum for XID wraparound would have to hit every page regardless.

There is one problem at this point. If we want to guarantee that there
are no tuples that XIDs are older than pg_class.relfrozenxid, we must scan
all pages for XID wraparound for every vacuums. So I used two thresholds
for treating XIDs, that is commented as follows. Do you have better ideas
for this point?

/*
* We use vacuum_freeze_min_age to determine whether a freeze scan is
* needed, but half vacuum_freeze_min_age for the actual freeze limits
* in order to prevent XID wraparound won't occur too frequently.
*/

Also, normal vacuums uses DSM and freeze-vacuum does not, so vacuums
sometimes take longer time than usual. Doesn't the surprise bother us?

Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center