curious vacuum full behavior - Mailing list pgsql-general

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

Thanks, Markus