From: | "Tomas Vondra" <tv(at)fuzzy(dot)cz> |
---|---|
To: | "MauMau" <maumau307(at)gmail(dot)com> |
Cc: | "Claudio Freire" <klaussfreire(at)gmail(dot)com>, "Merlin Moncure" <mmoncure(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org |
Subject: | Re: Could synchronous streaming replication really degrade the performance of the primary? |
Date: | 2012-05-10 12:20:02 |
Message-ID: | 5c3033fd2e913a6f10d02a4a3460ee15.squirrel@sq.gransy.com |
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Lists: | pgsql-performance |
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
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