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

2018-07-17 20:04 GMT-03:00 Mark Kirkwood <mark(dot)kirkwood(at)catalyst(dot)net(dot)nz>:
> 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).
>
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:

postgres=# \l
List of databases
Name | Owner | Encoding | Collate | Ctype |
Access privileges
---------------+----------+----------+-------------+-------------+-----------------------
postgres | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
template0 | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
=c/postgres +
| | | | |
postgres=CTc/postgres
template1 | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
=c/postgres +
| | | | |
postgres=CTc/postgres
tpch40gnorhdd | user1 | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
tpch40gnorssd | user1 | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
(5 rows)

postgres=#

After 7 query execution in a database tpch40gnorhdd I restart the DBMS
(/etc/init.d/pg101norssd restart and drop cache of the O.S.) and go to
execution test with the database tpch40gnorssd.
You think in this case there is pollution of shared_buffers?
Why do you think having O.S. on SSD is bad? Do you could explain better?

Best regards
[]`s Neto

> 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.
>>
>>
>