Thread: is there any adverse effect on DB if I set autovacuum scale factor to zero?

On 08/13/2018 11:07 AM, Raghavendra Rao J S V wrote:
> Hi All,
> 
> We are using postgres *9.2*  version on *Centos *operating system.  We 
> have around *1300+* tables.We have following auto vacuum settings are 
> enables. Still few of the tables(84 tables) which are always busy are 
> not vacuumed.Dead tuples in those tables are more than 5000.  Due to 
> that tables are bloating and observed few areas has performance 
> degradation.
> 

You don't say how large the tables are, so it's impossible to say 
whether 5000 dead tuples is excessive or not. IMHO it's a negligible 
amount and should not lead to excessive bloat or issues.

A certain amount of wasted is expected - it's a trade-off between 
immediate and delayed cleanup. If you delay the cleanup a bit, it's 
going to be more efficient overall.

It's also unclear why the tables are not vacuumed - it may easily be due 
to all the autovacuum workers being constantly busy, unable to cleanup 
all tables in a timely manner. In that case lowering the threshold is 
not going to help, on the contrary.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

On 08/13/2018 03:41 PM, Raghavendra Rao J S V wrote:
> 
> Hi Tomas,
> 
> Thank you very much for your response.
> 
> As we  know table becomes a candidate for autovacuum  process based on 
> below formula.
> 
> 
> *Autovacuum VACUUM thresold for a table = autovacuum_vacuum_scale_factor 
> * number of tuples + autovacuum_vacuum_threshold*
> 
> 
> 
> *Current settings in my database are as follows.*
> 
> 
> /autovacuum_vacuum_scale_factor = 0.1/
> 
> /autovacuum_vacuum_threshold = 40/
> 
> 
> 
> Due to above formula the dead tuples are accumulating based on the 
> number of live tuples as show below picture.
> 
> 
> 
> select relname,n_live_tup,n_dead_tup,(n_live_tup*.1+40) 
> expected_to_autovacuum,* from pg_stat_user_tables
> where  n_dead_tup>800
> order by n_live_tup desc
> limit 100;
> 
> 
> 
> 
> In order to avoid the dead tuples accumulation I wold like to change the 
> auto vacuum  settings in *"postgresql.conf"* as below.
> 
> /autovacuum_vacuum_scale_factor = 0.01/
> //autovacuum_vacuum_threshold = 100/
> /

OK, so the tables apparently have enough dead tuples to trigger vacuum. 
That mean the autovacuum throughput is insufficient to do all the 
cleanup. If you lower the scale factor, the amount of cleanup will 
*increase* (more tables being eligible for cleanup) making it less 
likely autovacuum can keep up.

You need to increase the throughtput, by increasing vacuum_cost_limit or 
something like that.

> *Kindly guide me your views. Does it cause any adverse effect on DB.*
> *

Well, it forces the database to do more stuff / more often, so it may 
have adverse impact, of course. It's hard to say if it's going to be a 
win overall, because we don't know how serious is the bloat.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

On 08/13/2018 04:24 PM, Ashu Pachauri wrote:
> + pgsql-general
> 
> Thanks and Regards,
> Ashu Pachauri
> 
> 
> ---------- Forwarded message ---------
> From: *Ashu Pachauri* <ashu210890@gmail.com <mailto:ashu210890@gmail.com>>
> Date: Mon, Aug 13, 2018 at 7:53 PM
> Subject: Re: is there any adverse effect on DB if I set autovacuum scale 
> factor to zero?
> To: <raghavendrajsv@gmail.com <mailto:raghavendrajsv@gmail.com>>
> 
> 
> The way I see *autovacuum_vacuum_scale_factor* is not in terms of 
> absolute number but as the percentage of any table that can consist of 
> updated / deleted tuples to make it eligible for vacuuming. A factor of 
> 0.1 ensures that your tables would be eligible for vacuuming if more 
> than 10% of the tuples are deleted/updated.
> 1. If you think that 10% is too high for you in terms of storage cost, 
> you can decrease the number or set it to zero. But, I would advise to 
> increase the value of *autovacuum_vacuum_threshold* to something 
> reasonable if you do that, otherwise you pay the CPU cost frequent 
> vacuuming across all tables.
> 2. However, if your issue is not the fixed 10% overhead but the lack of 
> throughput i.e. you see the number of deleted/updated tuples keeps 
> increasing in an unbounded fashion, the right way to deal with it is a) 
> Having higher value of *autovacuum_max_workers* b) lower value for 
> *autovacuum_naptime*.
> 

Increasing autovacuum_max_workers is unlikely to solve the issue with 
throughput, because all the workers are throttled together - there's a 
limit on the amount of work that can be done per second. Increasing the 
number of workers is akin to allowing more cars on a highway, but also 
lowering the speed limit.

You need to increase the limit on amount of work, and lowering naptime 
is one way to do that.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services