Re: [Postgres-xc-general] gtm network requirement and postgres dml support

[Mason S. <ms...@tr...>]

On Thu, Jan 16, 2014 at 11:18 AM, Ying He <yin...@ya...> wrote:

> hi, All,
>
> After reviewing
> http://postgres-xc.sourceforge.net/misc-docs/PG-XC_Architecture.pdf and
> reading mailing list. two things concerns me:1. the gtm network overhead 2.
> postgres dml support in plpgsql
>
> 1. Is 100Gbit/s network a prerequisite for the following setup?
>
>
No.


> If I plan to deploy a gtm master and slave on separate servers and 4 nodes
> of gtm_proxy, coordinator, datanodes and each will need to serve millions
> of transactions as daily volume.
>

Millions of transactions daily is not a problem.


>
> 2. Is there a doc that highlight which is currently not supported for
> postgres dml in plpgsql? Since the explain plan changes, i wonder how that
> affect postgres view predicate push and query performance.. The list will
> be important to consider postgres xc seriously because you can know what
> you need to change for your existing postgres setup.
>
>
IIRC, view rewriting turns into base relations internally post-parsing,
pre-planning, so push-down should work similarly as base tables.


> My main interest using postgres xc is its write scalability, all my tables
> will be replicated.
>

I do not recommend this.

With how many nodes? If you replicate to multiple nodes, two phase commit
will be used, so instead of getting write scalability, it will be slower
than a single native PostgreSQL instance. I think it only makes sense to
replicate all tables for read-only/read-mainly cases.

With Postgres-XC you get write scalability precisely because the tables are
sharded amongst multiple nodes, and you can keep those drives busy writing
across multiple servers.

If you are worried about redundancy, you can instance use streaming
replication so that you have individual node replicas. You can also put
these replicas on another server that contains the "master" of a data node.
The additional overhead is surprisingly small.




>
> It will be nice to see an updated PG-XC_Architecture.pdf which helps
> understand the internals.
>

There are some presentations out there that may provide more information
that you are looking for. One such slide deck is here:

http://www.slideshare.net/stormdb_cloud_database/postgresxc-write-scalable-postgresql-cluster


>
> Thank you for your help.
>
> best,
> Ying
>
>
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-- 
Mason Sharp

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