Subject: Re: Why HDD performance is better than SSD in this case
To: Neto pr <netopr9@gmail.com>, Fabio Pardi <f.pardi@portavita.eu>
Cc: pgsql-performance@lists.postgresql.org
References: 
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From: Mark Kirkwood <mark.kirkwood@catalyst.net.nz>
Message-ID: <d29dfb84-bb9a-d7c7-74eb-e263eae07901@catalyst.net.nz>
Date: Wed, 18 Jul 2018 11:04:44 +1200
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Ok, so dropping the cache is good.

How are you ensuring that you have one test setup on the HDDs and one on 
the SSDs? i.e do you have 2 postgres instances? or are you using one 
instance with tablespaces to locate the relevant tables? If the 2nd case 
then you will get pollution of shared_buffers if you don't restart 
between the HHD and SSD tests. If you have 2 instances then you need to 
carefully check the parameters are set the same (and probably shut the 
HDD instance down when testing the SSD etc).

I can see a couple of things in your setup that might pessimize the SDD 
case:
- you have OS on the SSD - if you tests make the system swap then this 
will wreck the SSD result
- you have RAID 0 SSD...some of the cheaper ones slow down when you do 
this. maybe test with a single SSD

regards
Mark

On 18/07/18 01:04, Neto pr wrote (note snippage):
> (echo 3> / proc / sys / vm / drop_caches;
>
> discs:
> - 2 units of Samsung Evo SSD 500 GB (mounted on ZERO RAID)
> - 2 SATA 7500 Krpm HDD units - 1TB (mounted on ZERO RAID)
>
> - The Operating System and the Postgresql DBMS are installed on the SSD disk.
>
>