Re: Rapid disk usage spikes when updating large tables with GIN indexes

[ please keep the list cc'd ]

Jonathan Marks <jonathanaverymarks(at)gmail(dot)com> writes:
> Thanks for your quick reply. Here’s a bit more information:
> 1) to measure the “size of the database” we run something like `select datname, pg_size_pretty(pg_database_size(datname)) from pg_database;` I’m not sure if this includes WAL size.
> 2) I’ve tried measuring WAL size with `select sum(size) from pg_ls_waldir();` — this also doesn’t budge.
> 3) Our current checkpoint_timeout is 600s with a checkpoint_completion_target of 0.9 — what does that suggest?

Hmph. Your WAL-size query seems on point, and that pretty much destroys
my idea about a WAL emission spike.

pg_database_size() should include all regular and temporary tables/indexes
in the named DB. It doesn't include WAL (but we've eliminated that), nor
cluster-wide tables such as pg_database (but those seem pretty unlikely
to be at issue), nor non-relation temporary files such as sort/hash temp
space. At this point I think we have to focus our attention on what might
be creating large temp files. I do not see anything in the GIN index code
that could do that, especially not if you have fastupdate off. I wonder
whether there is something about the particular bulk-insertion queries
you're using that could result in large temp files --- which'd make the
apparent correlation with GIN index use a mirage, but we're running out
of other ideas. You could try enabling log_temp_files to see if there's
anything to that.

In the grasping-at-straws department: are you quite sure that the extra
disk space consumption is PG's to begin with, rather than something
outside the database entirely?

regards, tom lane