Message-ID: <5c3033fd2e913a6f10d02a4a3460ee15.squirrel@sq.gransy.com>
In-Reply-To: <37AC2C75CBA4411F84F5F995EDF91503@maumau>
References: 
 <61B94EF1D7774715BC565DD84C731BDD@maumau><CAHyXU0w4vxULonSa+bJgALDK6gYHyvXcGj-VaZNE717ttkfXoA@mail.gmail.com><E7D1C27BDA0541B48AAA48ED4C0B94B5@maumau>
	<CAGTBQpYtod6ee6km0=C+U5yL9Wy5_9Bo7Nm719ymA42-SggLSA@mail.gmail.com>
	<37AC2C75CBA4411F84F5F995EDF91503@maumau>
Date: Thu, 10 May 2012 14:20:02 +0200
Subject: Re: Could synchronous streaming replication really degrade
	the performance of the primary?
From: "Tomas Vondra" <tv@fuzzy.cz>
To: "MauMau" <maumau307@gmail.com>
Cc: "Claudio Freire" <klaussfreire@gmail.com>,
	"Merlin Moncure" <mmoncure@gmail.com>, pgsql-performance@postgresql.org
User-Agent: SquirrelMail/1.4.22
MIME-Version: 1.0
Content-Type: text/plain;charset=utf-8
Importance: Normal
Content-Transfer-Encoding: quoted-printable

On 10 Kv=C4=9Bten 2012, 13:34, MauMau wrote:
> The workload is TPC-C-like write-heavy one; DBT-2. They compared the
> throughput of synchronous replication case against that of no replicati=
on
> case.
>
> Today, they told me that they ran the test on two virtual machines on a
> single physical machine. They also used pgpool-II in both cases. In
> addition, they may have ran the applications and pgpool-II on the same
> virtual machine as the database server.

So they've run a test that is usually I/O bound on a single machine? If
they've used the same I/O devices, I'm surprised the degradation was just
50%. If you have a system that can handle X IOPS, and you run two
instances there, each will get ~X/2 IOPS. No magic can help here.

Even if they used separate I/O devices, there are probably many things
that are shared and can become a bottleneck in a virtualized environment.

The setup is definitely very suspicious.

> It sounded to me that the resource is so scarce that concurrency was lo=
w,
> or
> your assumption may be correct. I'll hear more about their environment
> from
> them.
>
> BTW it's pity that I cannot find any case study of performance of the
> flagship feature of PostgreSQL 9.0/9.1, streaming replication...

There were some nice talks about performance impact of sync rep, for
example this one:

  http://www.2ndquadrant.com/static/2quad/media/pdfs/talks/SyncRepDurabil=
ity.pdf

There's also a video:

  http://www.youtube.com/watch?v=3DXL7j8hTd6R8

Tomas