Thread: curious vacuum full behavior

On Thu, 2021-02-04 at 10:03 +0000, Zwettler Markus (OIZ) wrote:
> I have 2 identical systems A + B.
> B being a clone of A.
> 
> The table pg_catalog.pg_largeobject was identical on both systems: 300GB in total size; 100GB bloated.
> 
> I did following on A:
> ð  vacuum full pg_catalog.pg_largeobject;
> (using the default maintenance_work_mem of 64MB)
> It took around 45 minutes and increased the diskspace by around 125% until the vacuum had been finished.
> 
> I did following on B:
> ð  set maintenance_work_mem = '256MB';
> ð  vacuum full pg_catalog.pg_largeobject;
> This took around 5 minutes. I don't know if the diskspace ever increased.
> 
> I was really surprised.
> Is there any explanation on this behavior?
> Is vacuum full heavily using on-disk sort areas if maintenance_work_mem is too low?
> 
> Postgres Version 9.6

VACUUM (FULL) will re-create the indexes too, and "maintenance_work_mem"
has an impact on index build speed.
But I have no explanation for such a large difference.

Yours,
Laurenz Albe
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