Comments on: Tuning Autovacuum in PostgreSQL and Autovacuum Internals

By: Zeeshan moin

Zeeshan moin — Tue, 12 Mar 2019 12:09:18 +0000

Hi Jobin,
can you please elaborate a little on autovacuum_vacuum_cost_limit. You had said it takes a default as 200.
I am trying to understand what does “cost” exactly mean here. Is that the no. of pages being scanned after every autovacuum wakeup by the worker process..? what does “cost” imply here.

By: Andrey Lepikhov

Andrey Lepikhov — Thu, 07 Feb 2019 04:20:27 +0000

In reply to Steve N.. It currently has no such settings.This can be explained by the following reasons: 1) most databases has 24/7 load with small oscillations; 2) Autovacuum is softly locks relations and cancel vacuuming if relation is used. On the over hand: not very difficult to develop an extension, which will launch background worker and control autovacuum according to some rules.

By: Andrey

Andrey — Thu, 07 Feb 2019 04:04:09 +0000

In reply to dharshanr. It is clean up all indexes, associated with the heap relation: Vacuum scans all index page-by page, deletes dead and unused tuples, removes empty pages.

By: dharshanr

dharshanr — Wed, 06 Feb 2019 18:02:41 +0000

Does Auto Vacuum clean up indexes as well or does it only work on the data files?

By: Steve N.

Steve N. — Fri, 25 Jan 2019 17:01:55 +0000

Are there any parameters that could be used to tell autovacuum to only operate during non peak hours? For example, we want autovacuum to be on, but not occur between 1PM and 4PM on the database? Or would we have to temporarily turn off autovacuum during that period?

By: Sreyas

Sreyas — Thu, 24 Jan 2019 08:28:40 +0000

In reply to Jobin Augustine. Hi Jobin, could you please give me your contact details, i have some queries regarding vacuum

By: Carlos Carmona

Carlos Carmona — Tue, 15 Jan 2019 22:20:27 +0000

You say “So, increasing the autovacuum_max_workers may delay the autovacuum execution for the currently running autovacuum workers. And increasing the autovacuum_vacuum_cost_limit may cause IO bottlenecks.”

So my question is, how can I tunning up my autovacuum process without create a bottlenecks or delay the autovacuum execution?

By: raviteja

raviteja — Thu, 29 Nov 2018 13:44:17 +0000

We are facing the issue with dead tuples where it’s not getting deleted even vacuum invokes and there is no lock on that table

By: Ian Warford (@iwarford)

Ian Warford (@iwarford) — Thu, 08 Nov 2018 17:06:07 +0000

In reply to Jobin Augustine. >Autovacuum won’t acquire locks >which can interfere with other >sessions doing SELECT/INSERT/ >UPDATE/DELETE. The lightweight >“cleanup lock” it acquires can >potentially can affect DDLs. Another question related to this. What are the long-term consequences from vacuum/autovacuum being unable to acquire that cleanup lock? Our experience showed that it was a brief exclusive lock on the table required to free up the space. However, on large busy tables, this wasn't possible for autovacuum to ever get. Has this been your experience?

By: Andrey

Andrey — Thu, 18 Oct 2018 03:00:20 +0000

In reply to Henry. I do not understand your task in general, but you can use WAL record XLOG_BTREE_VACUUM as a strong prove - it is log of index cleanup during vacuum.

By: Henry

Henry — Thu, 18 Oct 2018 02:37:04 +0000

thanks, i tried to add a stack dump to each XLogInsert call, and check if there are any that originates from autovacuum, but failed to find any instance to prove autovacuum calls are logged. possible to show the code path that dumps to wal? thanks

By: Andrey

Andrey — Thu, 18 Oct 2018 02:17:19 +0000

In reply to Henry. All VACUUM modifications are logging into WAL, block-by-block, and after crashing will be rollbacked

By: Henry

Henry — Thu, 18 Oct 2018 02:11:19 +0000

is autovacuum starting its modification as a transaction? what if autovacuum failed in the middle of cleaning dead tuples (like a power outage sth).

By: Jobin Augustine

Jobin Augustine — Tue, 04 Sep 2018 14:53:23 +0000

In reply to Ujang Jaenudin. I think that is a related topic but extension to what we discussed in this blog. Yes, you are correct, there are separate ring buffers to help autovacuum workers, let us consider what happens in a common workload. Autovacuum is not an aggressive backend process, it scans through tables from the candidate list one by one. so the pages required by autovacuum could be quite different from the current workload demands. There is a chance of getting the pages from current cache. However, the pages dirtied by autovacuum may not be needed again. To reduce the overall impact of autovacuum on shared buffers, the seperate ring buffer is maintained. There is a good chance that vacuum will almost always dirty pages once and never again, and flushing dirty pages back to the OS cache ASAP helps avoid a read-modify-write cycle if the page didn't leave the OS cache. vacuum related ring buffers are not currently tunable by parameters, But we can modify the source code and build the PostgreSQL. But we can expect an improved vacuum performance by doing so in servers with several GBs of RAM Source file : /backend/storage/buffer/freelist.c Function : GetAccessStrategy(BufferAccessStrategyType btype) However, There is a counter-argument from the community that if we optimize autovacuum to make it run faster, it may take away more IO bandwidth and cache. Still, many community members reported overall better performance in their workload. The takeaway is that same size won't fit for every workload. Do modifications with good amount of testing. When we work with PostgreSQL, always we have to keep in mind that we generally depends on OS level page cache to a good extent. General recommendation is to give 1/4 of RAM to shared buffers and rest to OS page cache. Any process accessing any memory page will have a side effect on the cache.

By: Ujang Jaenudin

Ujang Jaenudin — Tue, 04 Sep 2018 00:26:01 +0000

In reply to Jobin Augustine. "Yes, it can cause cache pollution just like any garbage collector." I heard there is "ring buffer" for special purposes i.e. vacuuming and also scanning big tables which 1/4 of shared buffer; could you please elaborate?

By: Jobin Augustine

Jobin Augustine — Mon, 13 Aug 2018 12:39:55 +0000

In reply to raghavendra. This is a mechanism to prevent autovacuum consuming excessive resource (I/O) on the server. By specifying this parameter we are saying to autovacuum workers to take a pause when the cost reaches a specific value.

By: Jobin Augustine

Jobin Augustine — Mon, 13 Aug 2018 12:25:53 +0000

In reply to Peter Zaitsev. Regarding the IO overhead, PostgreSQL is just delaying the I/O. Few databases move old snapshots/records to Undo area as part of transaction causing extra I/O as part of the transaction. But PostgreSQL takes an approach "Leave it there. don't move/copy anywhere". So PostgreSQL saves some IO there. But that saving is not permanent and ultimately it needs to do the garbage collection which is called vacuum. – Does autovacuum uses the cache (and hence can cause cache pollution) or does it use some different IO path ? Yes, it can cause cache pollution just like any garbage collector. However, The autovacuum algorithms are improved continuously from version to version and newer versions are able to avoid scanning pages wherever possible. The community effort is to bring it down to scan only those pages which are really modified. There is a high chance that such modified pages are there in the cache already. – What is about locking? Are there any locks which are being set by AutoVacum which can impact parallel queries beyond competition for IO resources? Autovacuum won't acquire locks which can interfere with other sessions doing SELECT/INSERT/UPDATE/DELETE. The lightweight "cleanup lock" it acquires can potentially can affect DDLs. However, Autovacuum is designed to be so gentle on the system, It has mechanisms to cancel itself, skip blocks or entire table itself, Running a newer version PostgreSQL is very important to get the best of it. Having said, Autovacuum is far away from perfectness. More efforts are underway to improve it further and address all edge cases.

By: Andrey

Andrey — Mon, 13 Aug 2018 11:32:55 +0000

In reply to Peter Zaitsev. Autovacuum acquire AccessExclusiveLock or ShareUpdateExclusiveLock on a vacuumed relation according to vacuum options.

By: raghavendra

raghavendra — Mon, 13 Aug 2018 09:56:28 +0000

What is the role of “autovacuum_vacuum_cost_limit”?

By: Peter Zaitsev

Peter Zaitsev — Mon, 13 Aug 2018 00:14:34 +0000

The overhead in terms of additional IO resource usage is clear. Couple of other questions

– Does autovacuum uses the cache (and hence can cause cache pollution) or does it use some different IO path ?

– What is about locking ? Are there any locks which are being set by AutoVacum which can impact parallel queries beyond competition for IO resources ?