Re: Visibility map, partial vacuums

Gregory Stark <stark(at)enterprisedb(dot)com> writes:

> Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
>
>> Gregory Stark wrote:
>>> 1) Raise autovacuum_max_freeze_age to 400M or 800M. Having it at 200M just
>>> means unnecessary full table vacuums long before they accomplish anything.
>>
>> It allows you to truncate clog. If I did my math right, 200M transactions
>> amounts to ~50MB of clog. Perhaps we should still raise it, disk space is cheap
>> after all.

Hm, the more I think about it the more this bothers me. It's another subtle
change from the current behaviour.

Currently *every* vacuum tries to truncate the clog. So you're constantly
trimming off a little bit.

With the visibility map (assuming you fix it not to do full scans all the
time) you can never truncate the clog just as you can never advance the
relfrozenxid unless you do a special full-table vacuum.

I think in practice most people had a read-only table somewhere in their
database which prevented the clog from ever being truncated anyways, so
perhaps this isn't such a big deal.

But the bottom line is that the anti-wraparound vacuums are going to be a lot
more important and much more visible now than they were in the past.

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Get trained by Bruce Momjian - ask me about EnterpriseDB's PostgreSQL training!