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

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.
>
>