Josh Berkus wrote:
> Shridhar,
>
> I was looking at the -V/-v and -A/-a settings in pgavd, and really don't
> understand how the calculation works. According to the readme, if I set -v
> to 1000 and -V to 2 (the defaults) for a table with 10,000 rows, pgavd would
> only vacuum after 21,000 rows had been updated. This seems wrong.
No. that is correct.
It is calculated as
threshold = base + scale*numebr of current rows
Which translates to
21,000 = 1000 + 2*1000
However I do not agree with this logic entirely. It pegs the next vacuum w.r.t
current table size which is not always a good thing.
I would rather vacuum the table at 2000 updates, which is what you probably want.
Furthermore analyze threshold depends upon inserts+updates. I think it should
also depends upon deletes for obvious reasons.
> Can you clear this up a little? I'd like to tweak these settings but can't
> without being better aquainted with the calculation.
What did you expected in above example? It is not difficult to tweak
pg_autovacuum calculations. For testing we can play around.
> Also, you may want to reverse your default ratio for Vacuum/analyze frequency.
> True, analyze is a less expensive operation than Vacuum, but it's also needed
> less often -- only when the *distribution* of data changes. I've seen
> databases where the optimal vacuum/analyze frequency was every 10 min/once
> per day.
OK vacuum and analyze thresholds are calculated with same formula as shown above
but with different parameters as follows.
vacthresh = vacbase + vacscale*ntuples
anathresh = anabase + anascale*ntuples
What you are asking for is
vacthresh = vacbase*vacscale
anathresh = anabase + anascale*ntuples
Would that tilt the favour the way you want? i.e. an analyze is triggered when a
fixed *percentage* of table changes but a vacuum is triggered when a fixed
*number of rows* are changed.
I am all for experimentation. If you have real life data to play with, I can
give you some patches to play around.
And BTW, this is all brain child of Mathew O.Connor(Correct? I am not good at
either names or spellings). The way I wrote pgavd originally, each table got to
get separate threshold..:-). That was rather a brute force approach.
Shridhar
