Re: Distributed transactions and asynchronous commit

Hi,

Il 17/07/2013 09:18, Xenofon Papadopoulos ha scritto:
> In the asynchronous commit documentation, it says:
>
> /The commands supporting two-phase commit, such as PREPARE
> TRANSACTION, are also always synchronous
> /
>
> Does this mean that all queries that are part of a distributed
> transaction are synchronous?
>
> In our databases we have extremely high disk I/O, I'm wondering if
> distributed transactions may be the reason behind it.

Distributed transactions are base on two-phase-commit (2PC) algorithms
for ensuring correct transaction completion, so are synchronous.
However, I think this is not the main reason behind your extremely high
disk I/O. You should check if your system is properly tuned to get the
best performances.
First of all, you could take a look on your PostgreSQL configurations,
and check if shared_memory is set properly taking into account your RAM
availability. The conservative PostgreSQL default value is 24 MB,
forcing system to exploit many disk I/O resources.
Aside from this, you could take a look if autovacuum is often triggered
(generating a large amount of I/O) in case of large use of
updates/inserts in your database.

Regards,

Giuseppe.

--
Giuseppe Broccolo - 2ndQuadrant Italy
PostgreSQL Training, Services and Support
giuseppe(dot)broccolo(at)2ndQuadrant(dot)it | www.2ndQuadrant.it