Re: Why HDD performance is better than SSD in this case

One more thought on this:

Query 9 does a lot pf sorting to disk - so there will be writes for that
and all the reads for the table scans. Thus the location of your
instance's pgsql_tmp directory(s) will significantly influence results.

I'm wondering if in your HDD test the pgsql_tmp on the *SSD's* is being
used. This would make the HDDs look faster (obviously - as they only
need to do reads now). You can check this with iostat while the HDD test
is being run, there should be *no* activity on the SSDs...if there is
you have just found one reason for the results being quicker than it
should be.

FWIW: I had a play with this: ran two version 10.4 instances, one on a
single 7200 rpm HDD, one on a (ahem slow) Intel 600p NVME. Running query
9 on the scale 40 databases I get:

- SSD 30 minutes

- HDD 70 minutes

No I'm running these on an a Intel i7 3.4 Ghz 16 GB RAM setup. Also both
postgres instances have default config apart from random_page_cost.

Comparing my results with yours - the SSD one is consistent...if I had
two SSDs in RAID0 I might halve the time (I might try this). However my
HDD result is not at all like yours (mine makes more sense to be
fair...would expect HDD to be slower in general).

Cheers (thanks for an interesting puzzle)!

Mark

On 18/07/18 13:13, Neto pr wrote:
>
> Dear Mark
> To ensure that the test is honest and has the same configuration the
> O.S. and also DBMS, my O.S. is installed on the SSD and DBMS as well.
> I have an instance only of DBMS and two database.
> - a database called tpch40gnorhdd with tablespace on the HDD disk.
> - a database called tpch40gnorssd with tablespace on the SSD disk.
> See below:
>
>