On Wed, May 9, 2012 at 3:58 PM, Merlin Moncure <mmoncure@xxxxxxxxx> wrote: > On Wed, May 9, 2012 at 8:06 AM, MauMau <maumau307@xxxxxxxxx> wrote: >> I've heard from some people that synchronous streaming replication has >> severe performance impact on the primary. They said that the transaction >> throughput of TPC-C like benchmark (perhaps DBT-2) decreased by 50%. I'm >> sorry I haven't asked them about their testing environment, because they >> just gave me their experience. They think that this result is much worse >> than some commercial database. > > I can't speak for other databases, but it's only natural to assume > that tps must drop. At minimum, you have to add the latency of > communication and remote sync operation to your transaction time. For > very short transactions this adds up to a lot of extra work relative > to the transaction itself. Actually I would expect 50% degradation if both databases run on identical hardware: the second instance needs to do the same work (i.e. write WAL AND ensure it reached the disk) before it can acknowledge. "When requesting synchronous replication, each commit of a write transaction will wait until confirmation is received that the commit has been written to the transaction log on disk of both the primary and standby server." http://www.postgresql.org/docs/9.1/static/warm-standby.html#SYNCHRONOUS-REPLICATION I am not sure whether the replicant can be triggered to commit to disk before the commit to disk on the master has succeeded; if that was the case there would be true serialization => 50%. This sounds like it could actually be the case (note the "after it commits"): "When synchronous replication is requested the transaction will wait after it commits until it receives confirmation that the transfer has been successful." http://wiki.postgresql.org/wiki/Synchronous_replication Kind regards robert -- remember.guy do |as, often| as.you_can - without end http://blog.rubybestpractices.com/ -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
