Re: Could synchronous streaming replication really degrade the performance of the primary?

On 10 Květen 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 replication
> 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 low,
> 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/SyncRepDurability.pdf

There's also a video:

  http://www.youtube.com/watch?v=XL7j8hTd6R8

Tomas