Date: Mon, 15 Jan 2018 19:58:25 -0600
From: Justin Pryzby <pryzby@telsasoft.com>
To: Neto pr <netopr9@gmail.com>
Cc: Fernando Hevia <fhevia@gmail.com>, "Georg H." <georg-h@silentrunner.de>,
	pgsql-performance@lists.postgresql.org
Subject: Re: HDD vs SSD without explanation
Message-ID: <20180116015825.GB3905@telsasoft.com>
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On Mon, Jan 15, 2018 at 05:19:59PM -0800, Neto pr wrote:
> >> Can you reproduce the speed difference using dd ?
> >> time sudo dd if=/dev/sdX of=/dev/null bs=1M count=32K
> >> skip=$((128*$RANDOM/32)) # set bs to optimal_io_size
> >
> > Still I would have expected somewhat similar results in the outcome, so yes,
> > it is weird that the SAS drive doubles the SSD performance. That is why I
> > think there must be something else going on during your tests on the SSD
> > server. It can also be that the SSD isn't working properly or you are
> > running an suboptimal OS+server+controller configuration for the drive.
> 
> Ok.
> 
> Can you help me to analyze the output of the command: dd if=/dev/sdX
> of=/dev/null bs=1M count=32K skip=$((128*$RANDOM/32)) # set bs to
> optimal_io_size

You should run the "dd" without the DB or anything else using the drive.  That
gets peformance of the drive, without the DB.

You should probably rerun the "dd" command using /dev/sdb1 if there's an
partition table on top (??).

I'm still wondering about these:

On Sun, Jan 14, 2018 at 09:09:41PM -0600, Justin Pryzby wrote:
> What about sdb partitions/FS?

> > > readahead?  blockdev --getra
> 
> > > If you're running under linux, maybe you can just send the output of:
> > > for a in /sys/block/sdX/queue/*; do echo "$a `cat $a`"; done
> > > or: tail /sys/block/sdX/queue/{minimum_io_size,optimal_io_size,read_ahead_kb,scheduler,rotational,max_sectors_kb,logical_block_size,physical_block_size}

Justin