Message-ID: <5294C0C4.1080003@dunslane.net>
Date: Tue, 26 Nov 2013 10:39:48 -0500
From: Andrew Dunstan <andrew@dunslane.net>
User-Agent: Mozilla/5.0 (X11; Linux x86_64;
 rv:17.0) Gecko/20130110 Thunderbird/17.0.2
MIME-Version: 1.0
To: Boszormenyi Zoltan <zb@cybertec.at>
CC: Heikki Linnakangas <hlinnakangas@vmware.com>,
 Lee Nguyen <leemobile@gmail.com>, pgsql-performance@postgresql.org
Subject: Re: Postgresql in a Virtual Machine
References: 
 <CAHghi5Cj_6KmwaUopF3y=vSjs3Q83VyXQy5DQhfLyP_Zt0yxbA@mail.gmail.com>
 <5293B0B9.1060200@vmware.com> <5294A77F.8050508@cybertec.at>
In-Reply-To: <5294A77F.8050508@cybertec.at>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: quoted-printable
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org


On 11/26/2013 08:51 AM, Boszormenyi Zoltan wrote:
> 2013-11-25 21:19 keltez=E9ssel, Heikki Linnakangas =EDrta:
>> On 25.11.2013 22:01, Lee Nguyen wrote:
>>> Hi,
>>>
>>> Having attended a few PGCons, I've always heard the remark from a few
>>> presenters and attendees that Postgres shouldn't be run inside a VM.=20
>>> That
>>> bare metal is the only way to go.
>>>
>>> Here at work we were entertaining the idea of running our Postgres=20
>>> database
>>> on our VM farm alongside our application vm's.  We are planning to=20
>>> run a
>>> few Postgres synchronous replication nodes.
>>>
>>> Why shouldn't we run Postgres in a VM?  What are the downsides? Does=20
>>> anyone
>>> have any metrics or benchmarks with the latest Postgres?
>>
>> I've also heard people say that they've seen PostgreSQL to perform=20
>> worse in a VM. In the performance testing that we've done in VMware,=20
>> though, we haven't seen any big impact. So I guess the answer is that=20
>> it depends on the specific configuration of CPU, memory, disks and=20
>> the software.
>
> We at Cybertec tested some configurations about 2 months ago.
> The performance drop is coming from the disk given to the VM guest.
>
> When there is a dedicated disk (pass through) given to the VM guest,
> PostgreSQL runs at a speed of around 98% of the bare metal.
>
> When the virtual disk is a disk file on the host machine, we've measured
> 20% or lower. The host used Fedora 19/x86_64 with IIRC a 3.10.x Linux=20
> kernel
> with EXT4 filesystem (this latter is sure, not IIRC). The effect was=20
> observed
> both under Qemu/KVM and Xen.
>
> The virtual disk was not pre-allocated, since it was the default setting,
> i.e. space savings preferred over speed. The figure might be better with
> a pre-allocated disk but the filesystem journalling done twice (both=20
> in the
> host and the guest) will have an effect.


Not-pre-allocated disk-file backed is just about the worst case in my=20
experience.

Try pre-allocated VirtIO disks on an LVM volume group - you should get=20
much better performance.

cheers

andrew


--=20
Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance