Horton

RE: reloption to prevent VACUUM from truncating empty pages at the end of relation

Tsunakawa, Takayuki <tsunakawa.takay@jp.fujitsu.com>

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From: "Tsunakawa, Takayuki" <tsunakawa.takay@jp.fujitsu.com>
To: 'Fujii Masao' <masao.fujii@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2018-04-19T05:32:44Z
Lists: pgsql-hackers

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  1. Add TRUNCATE parameter to VACUUM.

  2. Add vacuum_truncate reloption.

  3. Allow VACUUM to be run with index cleanup disabled.

From: Fujii Masao [mailto:masao.fujii@gmail.com]
> a very long time before accessing to the relation. Which would cause the
> response-time spikes, for example, I observed such spikes several times
> on
> the server with shared_buffers = 300GB while running the benchmark.

FYI, a long transaction took about 900 ms, while the average transaction response time was 150 ms or so.  (I'm working with Fujii-san in this performance benchmark.)


> Therefore, I'm thinking to propose $SUBJECT and enable it to avoid such
> spikes
> for that relation.

How about an integer variable to replace the following?

#define REL_TRUNCATE_FRACTION	16


> Also, first of all, if other transactions need to extend the relation
> (i.e., need new pages) as soon as VACUUM truncates the empty pages at the
> end,
> that truncation would not be so helpful for performance. In this case,
> the truncation and extension of the relation are unnecessarily repeated,
> which would decrease the performance. So, to alleviate this situation,
> $SUBJECT is useful, I think.

I wonder if fillfactor=50 would alleviate this situation.

Regards
Takayuki Tsunakawa