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

2018-07-17 22:13 GMT-03:00 Neto pr <netopr9(at)gmail(dot)com>:
> 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
>

+1 information about EVO SSD Samsung:

Model: 850 Evo 500 GB SATA III 6Gb/s -
http://www.samsung.com/semiconductor/minisite/ssd/product/consumer/850evo/

>> 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.
>>>
>>>
>>