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

Hi Neto,

You should list the SSD model also - there are pleinty of Samsung EVO
drives - and they are not professional grade.

Among the the possible issues, the most likely (from my point of view) are:

- TRIM command doesn't go through the RAID (which is really likely) - so
the SSD controller think it's full, and keep pushing blocks around to
level wear, causing massive perf degradation - please check this config
on you RAID driver/adapter

- TRIM is not configured on the OS level for the SSD

- Partitions is not correctly aligned on the SSD blocks

Without so little details on your system, we can only try to guess the
real issues

Nicolas

Nicolas CHARLES
Le 17/07/2018 à 15:19, Neto pr a écrit :
> 2018-07-17 10:04 GMT-03:00 Neto pr <netopr9(at)gmail(dot)com>:
>> Sorry.. I replied in the wrong message before ...
>> follows my response.
>> -------------
>>
>> Thanks all, but I still have not figured it out.
>> This is really strange because the tests were done on the same machine
>> (I use HP ML110 Proliant 8gb RAM - Xeon 2.8 ghz processor (4
>> cores), and POSTGRESQL 10.1.
>> - Only the mentioned query running at the time of the test.
>> - I repeated the query 7 times and did not change the results.
>> - Before running each batch of 7 executions, I discarded the Operating
>> System cache and restarted DBMS like this:
>> (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.
>>
> One more information.
> I used default configuration to Postgresql.conf
> Only exception is to :
> random_page_cost on SSD is 1.1
>
>
>> Best Regards
>> [ ]`s Neto
>>
>> 2018-07-17 1:08 GMT-07:00 Fabio Pardi <f(dot)pardi(at)portavita(dot)eu>:
>>> As already mentioned by Robert, please let us know if you made sure that
>>> nothing was fished from RAM, over the faster test.
>>>
>>> In other words, make sure that all caches are dropped between one test
>>> and another.
>>>
>>> Also,to better picture the situation, would be good to know:
>>>
>>> - which SSD (brand/model) are you using?
>>> - which HDD?
>>> - how are the disks configured? RAID? or not?
>>> - on which OS?
>>> - what are the mount options? SSD requires tuning
>>> - did you make sure that no other query was running at the time of the
>>> bench?
>>> - are you making a comparison on the same machine?
>>> - is it HW or VM? benchs should better run on bare metal to avoid
>>> results pollution (eg: other VMS on the same hypervisor using the disk,
>>> host caching and so on)
>>> - how many times did you run the tests?
>>> - did you change postgres configuration over tests?
>>> - can you post postgres config?
>>> - what about vacuums or maintenance tasks running in the background?
>>>
>>> Also, to benchmark disks i would not use a custom query but pgbench.
>>>
>>> Be aware: running benchmarks is a science, therefore needs a scientific
>>> approach :)
>>>
>>> regards
>>>
>>> fabio pardi
>>>
>>>
>>>
>>> On 07/17/2018 07:00 AM, Neto pr wrote:
>>>> Dear,
>>>> Some of you can help me understand this.
>>>>
>>>> This query plan is executed in the query below (query 9 of TPC-H
>>>> Benchmark, with scale 40, database with approximately 40 gb).
>>>>
>>>> The experiment consisted of running the query on a HDD (Raid zero).
>>>> Then the same query is executed on an SSD (Raid Zero).
>>>>
>>>> Why did the HDD (7200 rpm) perform better?
>>>> HDD - TIME 9 MINUTES
>>>> SSD - TIME 15 MINUTES
>>>>
>>>> As far as I know, the SSD has a reading that is 300 times faster than SSD.
>>>>
>>>> --- Execution Plans---
>>>> ssd 40g
>>>> https://explain.depesz.com/s/rHkh
>>>>
>>>> hdd 40g
>>>> https://explain.depesz.com/s/l4sq
>>>>
>>>> Query ------------------------------------
>>>>
>>>> select
>>>> nation,
>>>> o_year,
>>>> sum(amount) as sum_profit
>>>> from
>>>> (
>>>> select
>>>> n_name as nation,
>>>> extract(year from o_orderdate) as o_year,
>>>> l_extendedprice * (1 - l_discount) - ps_supplycost *
>>>> l_quantity as amount
>>>> from
>>>> part,
>>>> supplier,
>>>> lineitem,
>>>> partsupp,
>>>> orders,
>>>> nation
>>>> where
>>>> s_suppkey = l_suppkey
>>>> and ps_suppkey = l_suppkey
>>>> and ps_partkey = l_partkey
>>>> and p_partkey = l_partkey
>>>> and o_orderkey = l_orderkey
>>>> and s_nationkey = n_nationkey
>>>> and p_name like '%orchid%'
>>>> ) as profit
>>>> group by
>>>> nation,
>>>> o_year
>>>> order by
>>>> nation,
>>>> o_year desc
>>>>