Re: Is Synchronous Postgresql Replication Slower Than Asynchronous?

On 20 January 2012 21:17, John R Pierce <pierce(at)hogranch(dot)com> wrote:
> On 01/20/12 12:31 PM, Jerry Richards wrote:
>>
>> Is synchronous postgresql replication slower than asynchronous?  If so,
>> how much?  I am looking into database replication for a phone system, so the
>> response time is of concern.
>
>
> when a client issues a COMMIT on the master, synchronous by definition has
> to wait for the replication to be committed on the slave.  this will of
> course be slower, how much slower depends on far too many variables to give
> an estimate, including things like speed of both servers disk systems, speed
> of the network communications, data volume and transaction rate.

Synchronous replication is, by its very nature, slower than
asynchronous replication, and that has nothing to do with the
PostgreSQL implementation in particular. The whole point of using it
is to ensure that transactions have an additional level of durability.
The master must wait for confirmation from the standby before
indicating the transaction committed, so latency/distance becomes a
real concern.

Uniquely, the PostgreSQL implementation doesn't require that all
transactions be synchronous or asynchronous; you can dynamically
change that right down to the transaction level (or the client, or the
database...). You might like to make really important transactions
alone be replicated synchronously for super-durability. Many
applications that use this feature only use it for a small though
critical subset of transactions, like financial transactions.

--
Peter Geoghegan       http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training and Services