ITAGAKI Takahiro wrote:
Gregory Stark <st...@enterprisedb.com> wrote:
I don't think we can perfectly capture the meaning of these GUCs in the
name. I think our goal should be to avoid confusion between them.
I was thinking it would be clearer if the options which control *when*
autovacuum fires off a worker consistently had some action word in them like
"trigger" or "start" or "launch".
I think we need more explanations about those variables,
not only "how to work" but also "how to tune" them.
I feel they are un-tunable parameters.
Our documentation says:
| Larger values of these settings
| preserve transactional information longer, while smaller values increase
| the number of transactions that can elapse before the table must be
| vacuumed again.
i.e, we are explaining the variables only as "Larger is better",
but is it really true?
Yes, that is explicitly explained in the docs:
The sole disadvantage of increasing <varname>vacuum_freeze_table_age</>
and <varname>autovacuum_freeze_max_age</>
is that the <filename>pg_clog</> subdirectory of the database cluster
will take more space, because it must store the commit status for all
transactions back to the <varname>autovacuum_freeze_max_age</> horizon.
> ...
- What relation are there between autovacuum_freeze_max_age,
vacuum_freeze_min_age and vacuum_freeze_table_age? If we increase
one of them, should we also increase the others?
Yeah, that's a fair question. I'll try to work a doc patch to explain
that better.
- Is it ok to increase the variables to maximum values?
Are there any trade-off?
Disk space.
- Are there some conditions where whole-table-scanning vacuum is more
effective than vacuums using visibility map? If so, we should switch
to full-scan *automatically*, without relying on user configurations.
Hmm, the only downside I can see is that skipping a page here and there
could defeat the OS read-ahead. Perhaps we should call
posix_fadvise(SEQUENTIAL) to compensate. Or, we could modify the logic
to only skip pages when there's at least N consecutive pages that can be
skipped.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers
