Thread: set autovacuum=off

I have a database where I virtually never delete and almost never do updates. (The updates might change in the future but for now it's okay to assume they never happen.) As such, it seems like it might be worth it to set autovacuum=off or at least make it so vacuuming hardly ever occurs. Actually, the latter is probably the more robust solution, though I don't know how to do that (hence me writing this list). I did try turning autovacuum off but got:

ERROR: parameter "autovacuum" cannot be changed now
SQL state: 55P02 

On 22 February 2012 23:50, Alessandro Gagliardi <alessandro@path.com> wrote:
> I have a database where I virtually never delete and almost never do
> updates. (The updates might change in the future but for now it's okay to
> assume they never happen.) As such, it seems like it might be worth it to
> set autovacuum=off or at least make it so vacuuming hardly ever occurs.
> Actually, the latter is probably the more robust solution, though I don't
> know how to do that (hence me writing this list). I did try turning
> autovacuum off but got:
>
> ERROR: parameter "autovacuum" cannot be changed now
> SQL state: 55P02
>
> Not sure what, if anything, I can do about that.

Autovacuum is controlled by how much of a table has changed, so if a
table never changes, it never gets vacuumed (with the exceptional case
being a forced vacuum freeze to mitigate the transaction id
wrap-around issue).  The settings which control this are
autovacuum_vacuum_threshold and autovacuum_vacuum_scale_factor.
Therefore it isn't necessary to disable autovacuum.

But if you are adamant about disabling it, you need to change it in
your postgresql.conf file and restart the server.

--
Thom

On 2/23/2012 6:34 AM, Thom Brown wrote:
> On 22 February 2012 23:50, Alessandro Gagliardi<alessandro@path.com>  wrote:
>> I have a database where I virtually never delete and almost never do
>> updates. (The updates might change in the future but for now it's okay to
>> assume they never happen.) As such, it seems like it might be worth it to
>> set autovacuum=off or at least make it so vacuuming hardly ever occurs.
>> Actually, the latter is probably the more robust solution, though I don't
>> know how to do that (hence me writing this list). I did try turning
>> autovacuum off but got:
>>
>> ERROR: parameter "autovacuum" cannot be changed now
>> SQL state: 55P02
>>
>> Not sure what, if anything, I can do about that.
>
> Autovacuum is controlled by how much of a table has changed, so if a
> table never changes, it never gets vacuumed (with the exceptional case
> being a forced vacuum freeze to mitigate the transaction id
> wrap-around issue).  The settings which control this are
> autovacuum_vacuum_threshold and autovacuum_vacuum_scale_factor.
> Therefore it isn't necessary to disable autovacuum.
>
> But if you are adamant about disabling it, you need to change it in
> your postgresql.conf file and restart the server.
>

Agreed, don't disable autovacuum.  It's not that demanding, and if you
do need it and forget to run it, it might cause you more problems.

I have a db that's on a VM that doesnt get hit very much.  I've noticed
IO is a little busy (we are talking small percents of percents less than
one) but still more that I thought should be happening on a db with next
to no usage.

I found setting autovacuum_naptime = 6min made the IO all but vanish.

And if I ever get a wild hair and blow some stuff away, the db will
clean up after me.

-Andy

I should have been more clear. I virtually never delete or do updates, but I insert a lot. So the table does change quite a bit, but only in one direction. 

On 23 February 2012 17:35, Alessandro Gagliardi <alessandro@path.com> wrote:
> I should have been more clear. I virtually never delete or do updates, but I
> insert a lot. So the table does change quite a bit, but only in one
> direction.

The same thing applies.  VACUUM cleans up dead tuples, which INSERTs
don't create, only DELETE and UPDATEs do.

--
Thom

Hm. Okay, so just to be perfectly clear, my database with all its INSERTs, but no DELETEs or UPDATEs should not be VACUUMing anyway, so disabling auto-vacuum is redundant (and possibly hazardous).

On 02/23/2012 09:35 AM, Alessandro Gagliardi wrote:

I should have been more clear. I virtually never delete or do updates, but I insert a lot. So the table does change quite a bit, but only in one direction. 

I was unable to disable autovacuum universally (due to the cant_change_runtime_param error) but I was able to disable it on individual tables. Still, I know this is heavy handed and sub-optimal. I tried set autovacuum_naptime='6min' but got the same 55P02 error. Should/can I set that per table?

I did look at autovacuum_vacuum_threshold and autovacuum_vacuum_scale_factor but couldn't make sense out of them. (Besides, I'd probably get the same 55P02 error if I tried to change them.)

See:
http://www.postgresql.org/docs/current/static/runtime-config-autovacuum.html

The documentation has information like "This parameter can only be set in the postgresql.conf file or on the server command line." that will tell you in advance which settings will fail when you attempt to set them through SQL statements.

But autovacuum is pretty smart about not vacuuming tables until reasonably necessary. And beware that autovacuum is also controlling when to analyze a table. Mass inserts are probably changing the characteristics of your table such that it needs to be analyzed to allow the planner to properly optimize your queries.

Have you identified that vacuum is actually causing a problem? If not, I'd leave it alone. The system tables have a lot of information on table vacuuming and analyzing:

select
    relname,
    last_vacuum,
    last_autovacuum,
    last_analyze,
    last_autoanalyze,
    vacuum_count,
    autovacuum_count,
    analyze_count,
    autoanalyze_count
from
   pg_stat_user_tables;

Cheers,
Steve

On Thu, Feb 23, 2012 at 10:01 AM, Steve Crawford <scrawford@pinpointresearch.com> wrote:

On Thu, Feb 23, 2012 at 10:38 AM, Alessandro Gagliardi
<alessandro@path.com> wrote:
> On Thu, Feb 23, 2012 at 10:01 AM, Steve Crawford
> <scrawford@pinpointresearch.com> wrote:
> So if it's not auto-vacuuming that's making my inserts so slow, what is it?
> I'm batching my inserts (that didn't seem to help at all actually, but maybe
> cause I had already turned off synchronous_commit anyway). I've gotten rid
> of a bunch of indices (especially those with low cardinality–that I did
> around the same time as disabling auto-vacuum, so that could account for the
> coincidental speed up). I'm not sure what else I could be doing wrong. It's
> definitely better than it was a few days ago, but I still see "LOG:
> duration: 77.315 ms statement: COMMIT" every minute or two.
>

Have you considered that you may have lock contention? Sampling
pg_locks may be illuminating; based on your description the lock
contention would be intermittent, so I wouldn't trust an n=1 test.

-p

--
Peter van Hardenberg
San Francisco, California
"Everything was beautiful, and nothing hurt." -- Kurt Vonnegut

On 2/23/2012 12:38 PM, Alessandro Gagliardi wrote:

> Does analyze increase the efficiency of inserts or just selects? (I
> assumed the latter.) Obviously, I will need to analyze sometimes, but

That depends on if you have triggers that are doing selects.  But in
general you are correct, analyze wont help inserts.

checkpoint_segments can help insert speed, what do you have that set to?

Also how you insert can make things faster too. (insert vs prepared vs COPY)

Also, if you have too many indexes on a table that can cause things to
slow down.

Your IO layer needs to be fast too.  Have you watched vmstat and iostat?

Have you read up on synchronous_commit?

-Andy

On 02/23/2012 10:38 AM, Alessandro Gagliardi wrote:

On Thu, Feb 23, 2012 at 10:01 AM, Steve Crawford <scrawford@pinpointresearch.com> wrote:

The documentation has information like "This parameter can only be set in the postgresql.conf file or on the server command line." that will tell you in advance which settings will fail when you attempt to set them through SQL statements.

Ah. I missed that. Sorry for asking stupid questions.

No problem and not stupid. With the manual running to hundreds of pages plus information on wikis and mailing-list histories spanning hundreds of thousands of messages sometimes knowing where to look is 90% of the battle.

 

But autovacuum is pretty smart about not vacuuming tables until reasonably necessary. And beware that autovacuum is also controlling when to analyze a table. Mass inserts are probably changing the characteristics of your table such that it needs to be analyzed to allow the planner to properly optimize your queries.

Okay, that makes more sense to me; because the stats would be changing quickly and so while vacuuming may not be necessary, analyzing would be. At the same time, I can't afford to analyze if it's causing my inserts to take over 50 ms. Something else I should add: if my selects are slow, that's annoying; but if my inserts are slow, that could be disastrous...

You need to rethink things a bit. Databases can fail in all sorts of ways and can slow down during bursts of activity, data dumps, etc. You may need to investigate some form of intermediate buffering.

...Apparently the last four columns don't exist in my database. As for the first four, that is somewhat illuminating....

Then you are not running a current version of PostgreSQL so the first step to performance enhancement is to upgrade. (As a general rule - there are occasionally specific cases where performance decreases.)

So if it's not auto-vacuuming that's making my inserts so slow, what is it? I'm batching my inserts (that didn't seem to help at all actually, but maybe cause I had already turned off synchronous_commit anyway).

How are you batching them? Into a temp table that is copied to the main table? As a bunch of insert statements within a single connection (saves probably considerable time due to eliminating multiple connection setups)? With one PREPARE and multiple EXECUTE (to save repeated planning time - I'm not sure this will buy you much for simple inserts, though)? With COPY (much faster as many records are inserted in a single statement but if one fails, all fail)?

And what is the 50ms limit? Is that an average? Since you are batching, it doesn't sound like you need every statement to complete in 50ms. There is always a tradeoff between overall maximum throughput and maximum allowed latency.

I've gotten rid of a bunch of indices (especially those with low cardinality–that I did around the same time as disabling auto-vacuum, so that could account for the coincidental speed up).

Yes, inserts require the indexes to be updated so they can slow down inserts and updates.

I'm not sure what else I could be doing wrong. It's definitely better than it was a few days ago, but I still see "LOG: duration: 77.315 ms statement: COMMIT" every minute or two.

That's a huge topic ranging from hardware (CPU speed, RAM, spindle-count, disk-type, battery-backed write caching), OS (you *are* running on some sort of *nix, right?), OS tuning, PG tuning, etc. Fortunately the biggest benefit comes from some basic tuning.

I recommend you abandon this thread as it presupposes a now seemingly incorrect cause of the problem and start a new one titled something like "Tuning for high insert rate" where you describe the problem you want to solve. See http://wiki.postgresql.org/wiki/Guide_to_reporting_problems for a good guide to the information that will be helpful in diagnosis.

Cheers,
Steve

I'm unable to make sense of pg_locks. The vast majority are locktype='transactionid', mode='ExclusiveLock', granted=t. There are some 'relation' locks with mode='RowExclusiveLock' and fewer with 'AccessShareLock'. I have no idea what I should be looking for here.

On Thu, Feb 23, 2012 at 11:07 AM, Andy Colson <andy@squeakycode.net> wrote:

On 2/23/2012 2:40 PM, Alessandro Gagliardi wrote:
>
>     checkpoint_segments can help insert speed, what do you have that set to?
>
> 40. Checking
> http://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server it looks
> like setting that as high as 256 would not necessarily be unreasonable.
> What do you think?

I'd say go slow.  Try a little bit and see if it helps.  I don't
actually have high insert rate problems, so I don't actually know from
experience.

>
>     Also how you insert can make things faster too. (insert vs prepared
>     vs COPY)
>
> I'm doing this all with INSERT. Is COPY that much faster? I don't know
> anything about prepared.

If you can batch multiple records then COPY is the fastest method.  (Of
course your triggers might be the cause for the slowness and not insert
speed).

Depending on the language you are using to insert records, you can
prepare a query and only send the arguments vs sending the entire sql
statement every time.

In pseudo-perl code I'd:
my $q = $db->prepare('insert into table(col1, vol2) values ($1, $2)');

$q->execute('one', 'two');
$q->execute('three', 'four');
$q->execute('five', 'six');

This is faster because the "insert..." is only sent over the wire and
parsed once.  Then only the arguments are sent for each execute.

Speed wise, I think it'll go:
1) slowest: individual insert statements
2) prepared statements
3) fastest: COPY

Again.. assuming the triggers are not the bottleneck.

Have you run an insert by hand with 'EXPLAIN ANALYZE'?

-Andy


>
>     Have you read up on synchronous_commit?
>
> Only a tiny bit. A couple people suggested disabling it since my
> database is being hosted on AWS so I did that. It seems a bit risky but
> perhaps worth it.
>

I would think they are running on battery backed IO, with boxes on UPS,
so I'd guess its pretty safe.  It would also depend on your commit size.
  If you are batching a million records into one commit, you might loose
all of them.

-Andy

On Thu, Feb 23, 2012 at 11:26 AM, Steve Crawford <scrawford@pinpointresearch.com> wrote:

On 02/23/2012 01:07 PM, Alessandro Gagliardi wrote:

> The second one (a bunch of insert statements within a single
> connection). As I mentioned above, I was going to try the temp table
> thing, but that wasn't fast enough. COPY might be my next attempt.
insert into...;
insert into...;
insert into...;
... is really (ignoring statement preparation time):
begin;
insert into...;
commit;
begin;
insert into...;
commit;
begin;
insert into...;
commit;

It's possible that you might get a nice boost by wrapping the inserts
into a transaction:
begin;
insert into...;
insert into...;
insert into...;
...
commit;

This only requires all that disk-intensive stuff that protects your data
once at the end instead of 1000 times for you batch of 1000.

COPY is even better. I just ran a quick test by restoring a table on my
desktop hacking db (untuned, few years old PC, single SATA disk, modest
RAM and lots of resource competition). The 22+ million rows restored in
282 seconds which is a rate somewhat north of 78,000 records/second or
about 0.13ms/record.

You may want to eliminate that trigger, which only seems to exist to
silence errors from uniqueness violations, and copy the incoming data
into a temp table then move the data with a variant of:
INSERT INTO main_table (SELECT ... FROM incoming_table WHERE NOT EXISTS
((SELECT 1 from main_table WHERE ...))

Cheers,
Steve

On Thu, Feb 23, 2012 at 1:37 PM, Steve Crawford <scrawford@pinpointresearch.com> wrote:

On Thu, Feb 23, 2012 at 1:11 PM, Peter van Hardenberg <pvh@heroku.com> wrote:

On Thu, Feb 23, 2012 at 1:07 PM, Alessandro Gagliardi
<alessandro@path.com> wrote:
>>
>> ...Apparently the last four columns don't exist in my database. As for the
>> first four, that is somewhat illuminating....
>>
>> Then you are not running a current version of PostgreSQL so the first step
>> to performance enhancement is to upgrade. (As a general rule - there are
>> occasionally specific cases where performance decreases.)
>>
> We're using 9.0.6. Peter, how do you feel about upgrading? :)
>

9.1's in beta; we're working on writing an upgrade system before
calling it GA, but it works fine. Feel free.

My hunch is still that your issue is lock contention.

> No, not average. I want to be able to do 100-200 INSERTs per second (90% of
> those would go to one of two tables, the other 10% would go to any of a
> couple dozen tables). If 1% of my INSERTs take 100 ms, then the other 99%
> must take no more than 9 ms to complete.
> ...actually, it occurs to me that since I'm now committing batches of 1000,
> a 100ms latency per commit wouldn't be bad at all! I'll have to look into
> that.... (Either that or my batching isn't working like I thought it was.)
>

We have many customers who do much more than this throughput, though
I'm not sure what level of resourcing you're current at. You might
consider experimenting with a larger system if you're having
performance problems.

Peter

yup there is. the parameter checkpoint_segments does not require a restart of the server, just a reload :)

you might also consider increasing the wal_buffers value if it's still the default (64KB)

BR

On Thu, Feb 23, 2012 at 3:28 PM, Alessandro Gagliardi
<alessandro@path.com> wrote:
> I'm unable to make sense of pg_locks. The vast majority are
> locktype='transactionid', mode='ExclusiveLock', granted=t. There are some
> 'relation' locks with mode='RowExclusiveLock' and fewer with
> 'AccessShareLock'. I have no idea what I should be looking for here.

If you have lock contention, you'll see locks with granted='f', at
least from time to time.  Those are the ones you want to worry about.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company