RE: How can i be certain autovacuum is causing reuse if table still grows

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От Sidney Aloisio Ferreira Pryor
Тема RE: How can i be certain autovacuum is causing reuse if table still grows
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Msg-id CP2PR80MB0707B010FEDFE1298BDDEBF18C730@CP2PR80MB0707.lamprd80.prod.outlook.com
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Ответ на Re: How can i be certain autovacuum is causing reuse if table still grows  (Keith Fiske <keith.fiske@crunchydata.com>)
Ответы RE: How can i be certain autovacuum is causing reuse if table still grows  (Sidney Aloisio Ferreira Pryor <sidney@tjrj.jus.br>)
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Thank you very much for this complete explanation and opinion, Keith.

The first goal of publishing this mail on the list was to answer how to measure precisely free space: pgstattuple is the correct one.
The second goal is running so far: manual vacuum to accomplish reuse of that large free area. (i will post a pgstattuple of the table when this vacuum ends)
And the third goal is to free and downsize this huge allocation of space: schedule an outage to run vacuum full.

Again i am truly grateful for your precious help.
And i will post here Pg_total_relation_size before and after running vacuum full when we schedule that outage.

Best Regards,
Sidney Pryor

De: Keith Fiske <keith.fiske@crunchydata.com>
Enviado: terça-feira, 28 de julho de 2020 10:55
Para: Sidney Aloisio Ferreira Pryor <sidney@tjrj.jus.br>
Cc: Ron <ronljohnsonjr@gmail.com>; pgsql-admin@lists.postgresql.org <pgsql-admin@lists.postgresql.org>
Assunto: Re: How can i be certain autovacuum is causing reuse if table still grows
 
Again, I would caution against going with partitioning until you can first try and tune the table itself. I have quite a bit of experience with it as I'm the author of pg_partman so you can see I have nothing against partitioning itself, but I'm only for using it when it's necessary. If you decide to go that route, it would be a good tool to help you automate much of the maintenance process of partitioning either by time or integer.


I say this because, while it could possibly help with making vacuum more efficient, there are many caveats to partitioning. Since you're on 9.6, there is no native partitioning, so you'd have to do trigger based partitioning which has a HUGE impact on write performance (at least 2x slower if not worse). Even with native partitioning in 10+, there is a performance penalty during tuple routing and you would want to test that impact. Especially if you are doing updates which would move data across child partitions (updates that move data prior to PG10 are very, very complicated and not supported by pg_partman). Also, if you have any unique keys to maintain, they are not enforced across the partition set, and only supported in native if they are also part of the partition key itself. You're also just adding to the general complexity of your database overall. 

I would in general say the primary reason you would want to consider partitioning in PG is if you're trying to expire old data out. Dropping a table is much more efficient than running large/many DELETE statements due to the very issues you're encountering now: bloat. But it sounds like you are not expiring data, so you also have to take into account how many partitions you will have long term. It wasn't until PG12 that having over just even a few hundred partitions could have major performance impact overall. So, before even considering partitioning, you would want to get onto the latest major release.

Partitioning can help with vacuum and general maintenance, but that's really only when the actual live data in a single table starts getting very, very large. And in that case you may want to first consider your hardware resources before going down the road of partitioning since that may be the better benefit long term. You've got nearly 2.3TB of space to clean up in this table, so, again, you want to solve that problem first. If you had that much free space before this vacuum finished, you likely have even more now after it finishes. And just note that a regular vacuum is likely not going to clean up this space. It may clean up some, but vacuum only returns space to the system in very specific circumstances (the tail-end pages are empty). You could use something like pg_repack or pg_squeeze to do this without a long outage, but with bloat this size, you may run into issues with how long those tools would need to run. Especially if the table is still in use. So, again, I would recommend an outage to do a VACUUM FULL which will lock and rewrite the table. I'd be curious to see what your total size of the entire table before and after is. The following function will give that total size (table + indexes)

select pg_total_relation_size('public.mytable');
 
So, while partitioning could possibly be the answer long term, I would not recommend it until you've cleaned up the existing bloat on this table and its indexes and try to tune autovacuum run more efficiently. If that works, you've saved yourself a great deal of complexity. And again, you'd definitely want to get on the latest major version of PG where partitioning has improved tremendously, so if a major version upgrade isn't in the works for a while, I wouldn't even consider it.

--
Keith Fiske
Senior Database Engineer
Crunchy Data - http://crunchydata.com


On Tue, Jul 28, 2020 at 6:18 AM Sidney Aloisio Ferreira Pryor <sidney@tjrj.jus.br> wrote:
Thank you Ron
I would appreciate if you could share a good documentation or review link about partitioning tables on postgres. 

Enviado do meu iPhone

Em 28 de jul de 2020, à(s) 02:16, Ron <ronljohnsonjr@gmail.com> escreveu:


> All lines even old ones might be updated by our application.

Ah.  Even so, there are benefits to maintaining "lots of smaller chunks" instead of One Giant Table: "pg_dump --jobs=X" will run faster, for example, when there are more small-to-mid-sized tables instead of A Few Giant Tables.  Ditto "reindexdb --jobs=X". 

Given that even "old" records get updated, I'd look into some other key that you can "naturally" partition on.

On 7/27/20 10:54 PM, Sidney Aloisio Ferreira Pryor wrote:
Thank you Ron.
Yes there is a column id in which value is provided by a sequence.

But it was not clear to me exactly how to partition?
The idea is to cover a limited range of "id" so there will be less lines to vacuum on each operation?

Because yet we will need eventually to vacuum all others.
All lines even old ones might be updated by our application.

De: Ron <ronljohnsonjr@gmail.com>
Enviado: terça-feira, 28 de julho de 2020 00:39
Para: pgsql-admin@lists.postgresql.org <pgsql-admin@lists.postgresql.org>
Assunto: Re: How can i be certain autovacuum is causing reuse if table still grows
 
On 7/27/20 5:19 PM, Keith Fiske wrote:
[snip]
> Also, I would not recommend partitioning simply to improve vacuuming.
> Especially if extensive tuning hasn't been tried first. Most times you can
> get per-table tuning working well enough to get autovacuum running
> properly. Especially on 9.6 and even more-so on PG11, where autovacuum has
> itself been improved.

SIMPLY to improve vacuum performance?  No.  But there are reasons that
partitioning was "invented", and minimizing the work needed to be done on
the whole of a Very Large and Rapidly Growing table is one of them.

--
Angular momentum makes the world go 'round.



--
Angular momentum makes the world go 'round.


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