From: Mark Kirkwood <mark.kirkwood@catalyst.net.nz>
To: Neto pr <netopr9@gmail.com>
To: Fabio Pardi <f.pardi@portavita.eu>
Cc: pgsql-performance@lists.postgresql.org
Subject: Re: Why HDD performance is better than SSD in this case
Date: Wed, 18 Jul 2018 11:04:44 +1200
Message-ID: <d29dfb84-bb9a-d7c7-74eb-e263eae07901@catalyst.net.nz> (raw)
In-Reply-To: <CA+wPC0M=e5ZXd1NF1nz6mYfmPgys89vB4A74GmxnnK+KpA4_XA@mail.gmail.com>
References: <CA+wPC0PsWacZpNX2OfRSSauUg9iNa7tReJU0oTLnM7K8PFFt6A@mail.gmail.com>
<01bbbc88-2a47-f078-ea40-ab2c1acafa1f@portavita.eu>
<CA+wPC0M=e5ZXd1NF1nz6mYfmPgys89vB4A74GmxnnK+KpA4_XA@mail.gmail.com>
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.
>
>
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To: pgsql-performance@postgresql.org
Cc: mark.kirkwood@catalyst.net.nz, netopr9@gmail.com, f.pardi@portavita.eu, pgsql-performance@lists.postgresql.org
Subject: Re: Why HDD performance is better than SSD in this case
In-Reply-To: <d29dfb84-bb9a-d7c7-74eb-e263eae07901@catalyst.net.nz>
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