Thread: "VACUUM FULL ANALYZE" vs. Autovacuum Contention

Hello,

(Apologies for any possible duplication of this email.)

(Also, apologies if this is an obvious question.  I have gone through the archives without seeing something that directly ties to this.)

We are running Postgresql on a 64b RHEL5.2 64b server.  "Uname -a":
--------------Linux xxxxxxx 2.6.18-92.el5 #1 SMP Tue Apr 29 13:16:15 EDT 2008 x86_64 x86_64 x86_64 GNU/Linux

We have autovacuum enabled with the following settings:

autovacuum_naptime = 30s
autovacuum_vacuum_threshold = 200
autovacuum_vacuum_scale_factor = 0.5
autovacuum_vacuum_cost_delay = 10

In addition to autovacuuming, each day, early, in the morning, we run a full vacuum, like this: "vacuumdb --all --full --analyze".  We do not have any special variable set for vacuum in postgresql.conf.

The problem is that once or twice a week, the "vacuum full analyze" seems to cancel out the autovacuum that has already started at the same time.  E.g.,

-------------2011-05-07 03:51:04.959 EDT--[unknown]-[unknown] [3348]LOG:  connection received: host=##.##.##.## port=60470
-------------2011-05-07 03:51:04.959 EDT-##.##.##.##-xxxx-xxxx [3348]LOG:  connection authorized: user=xxxx database=XXXX
-------------2011-05-07 03:51:04.961 EDT-##.##.##.##-xxxx-xxxx [3348]LOG:  statement: VACUUM FULL ANALYZE;
-------------...   
-------------2011-05-07 03:51:10.733 EDT--- [19879]ERROR:  canceling autovacuum task
-------------2011-05-07 03:51:10.733 EDT--- [19879]CONTEXT: automatic vacuum of table "xxxx.xxx.xxxx"
-------------...
-------------2011-05-07 03:52:48.918 EDT-##.##.##.##-xxxx-xxxx [3348]LOG:  duration: 103957.270 ms
-------------2011-05-07 03:52:48.920 EDT-##.##.##.##-xxxx-xxxx [3348]LOG:  disconnection: session time: 0:01:43.961 user=xxxx database=xxxx host=##.##.##.## port=60470

We would like to eliminate this error.  A bigger problem is that sometimes it seems like autovacuum wins out over "vacuum full analyze".  This tends to result in a hung job on our client, with other ensuing complications.

* Our basic question is what method we might be able to use to prevent either of these jobs from canceling.  What we would like is, instead of autovacuum canceling, it rather always defers to "vacuum full analyze" job, waiting for it to complete.

I am guessing that we can do the above by setting the "autovacuum_vacuum_cost_limit" to a fairly high value (rather than it not being set at all, as it is right now, and thus inheriting the "200" default value from vacuum_cost_limit).  Does that sound right?  (If, what might be a good value to set?)  Or perhaps there is a more foolproof way of doing this that does not rely upon guesswork?

Any suggestions at all would be most welcome!

Daniel C.

On Thu, Jul 7, 2011 at 2:30 PM, D C <ptradingcom@gmail.com> wrote:
> Hello,
>
> (Apologies for any possible duplication of this email.)
>
> (Also, apologies if this is an obvious question.  I have gone through the
> archives without seeing something that directly ties to this.)
>
> We are running Postgresql on a 64b RHEL5.2 64b server.  "Uname -a":
> --------------Linux xxxxxxx 2.6.18-92.el5 #1 SMP Tue Apr 29 13:16:15 EDT
> 2008 x86_64 x86_64 x86_64 GNU/Linux
>
> We have autovacuum enabled with the following settings:
>
> autovacuum_naptime = 30s
> autovacuum_vacuum_threshold = 200
> autovacuum_vacuum_scale_factor = 0.5
> autovacuum_vacuum_cost_delay = 10
>
> In addition to autovacuuming, each day, early, in the morning, we run a full
> vacuum, like this: "vacuumdb --all --full --analyze".

Why?

On 07/07/2011 04:30 PM, D C wrote:
>
> autovacuum_naptime = 30s
> autovacuum_vacuum_threshold = 200
> autovacuum_vacuum_scale_factor = 0.5
> autovacuum_vacuum_cost_delay = 10
>

These are slightly strange settings.  How did you come up with them?
The autovacuum_vacuum_scale_factor being so high is particularly
dangerous.  If anything, you should be reducing that from its default of
0.2, not increasing it further.

> In addition to autovacuuming, each day, early, in the morning, we run
> a full vacuum, like this: "vacuumdb --all --full --analyze".  We do
> not have any special variable set for vacuum in postgresql.conf.
>

VACUUM FULL takes an exclusive lock on the table while it runs, and it
extremely problematic for several other reasons too.  See
http://wiki.postgresql.org/wiki/VACUUM_FULL for more information.

You didn't mention your PostgreSQL version so I can't be sure exactly
how bad of a problem you're causing with this, but you should almost
certainly stop doing it.


> The problem is that once or twice a week, the "vacuum full analyze"
> seems to cancel out the autovacuum that has already started at the
> same time.  E.g.,
>

Yes.  VACUUM FULL needs to take a large lock on the table, and it will
kick out autovacuum in that case, and cause countless other trouble
too.  And if the VACUUM FULL is already running, other things will end
up getting stuck waiting for it, and all sorts of locking issues can
come out of that.

You should remove the "--full" from your daily routine, reduce
autovacuum_vacuum_scale_factor back to a reasonable number again, and
see how things go after that.  You're trying to use PostgreSQL in a way
it's known not to work well right now.

> I am guessing that we can do the above by setting the
> "autovacuum_vacuum_cost_limit" to a fairly high value (rather than it
> not being set at all, as it is right now, and thus inheriting the
> "200" default value from vacuum_cost_limit).
>

The cost limit has nothing to do with the issue you're seeing.  It
adjust how much work autovacuum does at any moment in time, it isn't
involved in any prioritization.

--
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
Comprehensive and Customized PostgreSQL Training Classes:
http://www.2ndquadrant.us/postgresql-training/

That's a great point about autovacuum_vacuum_scale_factor; I will lower the value there to 0.2 and see if autovacuum starts doing a better job.  (We use Postgresql 8.3.5 currently, by the way.)

Thanks for the notes and the useful page link on "vacuum full".  We are running "vacuum full" primarily because a number of tables in our database have a very large amount of data added to them during each day, all of which is deleted in one large series of "delete from" statements early in the morning before we perform the vacuum.  Comments like the one here (http://www.postgresql.org/docs/9.0/static/routine-vacuuming.html) led us to think that with this type of situation (very large deletes daily) autovacuum would not in the end be sufficient over the long run.

That said, it sounds like if we switched to daily "trucates" of each table (they can be purged entirely each day) rather than "delete froms", then there truly would not be any reason to use "vacuum full".  Does that sound plausible?

Thanks again,

Daniel

On 07/08/2011 12:46 PM, D C wrote:
> That said, it sounds like if we switched to daily "trucates" of each
> table (they can be purged entirely each day) rather than "delete
> froms", then there truly would not be any reason to use "vacuum
> full".  Does that sound plausible?


That's exactly right.  If you can re-arrange this data to be truncated
instead of deleted, this entire problem should go away.  There is also a
nice optimization you should know about; if you do this:

BEGIN;
TRUNCATE t;
COPY t FROM ...
COMMIT;

In single-node systems (no standby slave), this can work much faster
than a normal load.  It's able to skip the pg_xlog WAL writes in this
situation.

--
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
Comprehensive and Customized PostgreSQL Training Classes:
http://www.2ndquadrant.us/postgresql-training/