In our sharded cluster project we are trying to use logical relication for providing HA (maintaining redundant shard copies). Using asynchronous logical replication has not so much sense in context of HA. This is why we try to use synchronous logical replication. Unfortunately it shows very bad performance. With 50 shards and level of redundancy=1 (just one copy) cluster is 20 times slower then without logical replication. With asynchronous replication it is "only" two times slower.
As far as I understand, the reason of such bad performance is that synchronous replication mechanism was originally developed for streaming replication, when all replicas have the same content and LSNs. When it is used for logical replication, it behaves very inefficiently. Commit has to wait confirmations from all receivers mentioned in "synchronous_standby_names" list. So we are waiting not only for our own single logical replication standby, but all other standbys as well. Number of synchronous standbyes is equal to number of shards divided by number of nodes. To provide uniform distribution number of shards should >> than number of nodes, for example for 10 nodes we usually create 100 shards. As a result we get awful performance and blocking of any replication channel blocks all backends. So my question is whether my understanding is correct and synchronous logical replication can not be efficiently used in such manner. If so, the next question is how difficult it will be to make synchronous replication mechanism for logical replication more efficient and are there some plans to work in this direction? -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
