Re: Distributed transactions and asynchronous commit

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.

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Giuseppe Broccolo - 2ndQuadrant Italy
PostgreSQL Training, Services and Support
giuseppe.broccolo@2ndQuadrant.it | www.2ndQuadrant.it