Re: HDD vs SSD without explanation - Mailing list pgsql-performance

2018-01-14 13:40 GMT-08:00 Justin Pryzby <pryzby@telsasoft.com>:
> On Sun, Jan 14, 2018 at 12:44:00PM -0800, Neto pr wrote:
>> Dear all
>>
>> Someone help me analyze the two execution plans below (Explain ANALYZE
>> used), is the  query 9 of TPC-H benchmark [1].
>>
>> I'm using a server HP Intel Xeon 2.8GHz/4-core - Memory 8GB HDD SAS 320GB
>> 15 Krpm AND SSD Sansung EVO 500GB.
>>
>> I think maybe the execution plan is using more write operations, and so the
>> HDD SAS 15Krpm has been faster.
>
> The query plan is all garbled by mail , could you resend?  Or post a link from
> https://explain.depesz.com/
>
> To see if the query is causing many writes (due to dirty pages, sorts, etc),
> run with explain(analyze,buffers)
>
> But from what I could tell, your problems are here:
>
> ->  Parallel Seq Scan on lineitem  (cost=0.00..5861332.93 rows=100005093 width=41) (actual TIME=3.494..842667.110
rows=80004097loops=3)
 
> vs
> ->  Parallel Seq Scan on lineitem  (cost=0.00..5861333.40 rows=100005140 width=41) (actual TIME=41.805..224438.909
rows=80004097loops=3)
 
>
> ->  Seq Scan on partsupp (cost=0.00..1052983.08 rows=31999708 width=22) (actual TIME=0.033..228828.149 rows=32000000
loops=3)
> vs
> ->  Seq Scan on partsupp  (cost=0.00..1052934.38 rows=31994838 width=22) (actual TIME=0.037..37865.003 rows=32000000
loops=3)
>
> 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
>
> What OS/kernel are you using?  LVM?  filesystem?  I/O scheduler?  partitions?
> readahead?  blockdev --getra

OS = Debian 8 64bits - 3.16.0-4

See below the Disk FileSystem --------------------------------
root@hp2ml110deb:/# fdisk -l
Disk /dev/sda: 931.5 GiB, 1000204886016 bytes, 1953525168 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
Disklabel type: gpt
Disk identifier: 26F5EB21-30DB-44E4-B9E2-E8105846B6C4

Device          Start        End    Sectors   Size Type
/dev/sda1        2048    1050623    1048576   512M EFI System
/dev/sda2     1050624 1937274879 1936224256 923.3G Linux filesystem
/dev/sda3  1937274880 1953523711   16248832   7.8G Linux swap

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
----------------------------------------------------------------------------

The DBMS and tablespace of users is installed in /dev/sdb  SSD.

> 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