Date: Sun, 14 Jan 2018 21:09:41 -0600
From: Justin Pryzby <pryzby@telsasoft.com>
To: Neto pr <netopr9@gmail.com>
Cc: postgres performance list <pgsql-performance@postgresql.org>
Subject: Re: HDD vs SSD without explanation
Message-ID: <20180115030941.GG32403@telsasoft.com>
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On Sun, Jan 14, 2018 at 06:25:40PM -0800, Neto pr wrote:
> > The query plan is all garbled by mail , could you resend?  Or post a link from
> > https://explain.depesz.com/

On Sun, Jan 14, 2018 at 06:36:02PM -0800, Neto pr wrote:
> I was not able to upload to the site, because I'm saving the execution
> plan in the database, and when I retrieve it, it loses the line breaks,

That's why it's an issue for me, too..

> > What OS/kernel are you using?  LVM?  filesystem?  I/O scheduler?  partitions?
> 
> See below the Disk FileSystem --------------------------------
> root@hp2ml110deb:/# fdisk -l
> Disk /dev/sda: 931.5 GiB, 1000204886016 bytes, 1953525168 sectors
> 
> Disk /dev/sdb: 465.8 GiB, 500107862016 bytes, 976773168 sectors
> Units: sectors of 1 * 512 = 512 bytes
> Sector size (logical/physical): 512 bytes / 512 bytes
> I/O size (minimum/optimal): 512 bytes / 512 bytes
> ----------------------------------------------------------------------------
What about sdb partitions/FS?

On Sun, Jan 14, 2018 at 06:25:40PM -0800, Neto pr wrote:
> The DBMS and tablespace of users is installed in /dev/sdb  SSD.

Is that also a temp_tablespace ?  Or are your hashes spilling to HDD instead ?

Group Key: nation.n_name, (date_part(_year_::text, (orders.o_orderdate)::timestamp without time zone))
Buffers: shared hit=3773802 read=7120852, temp read=3550293 written=3541542

Are your SSD being used for anything else ?

What about these?

> > 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}

> > 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
> >
> > Or: bonnie++ -f -n0

Justin