Hi, On 2017-10-07 22:39:09 +0300, konstantin knizhnik wrote: > 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?
This seems to be a question that is a) about a commercial project we don't know much about b) hasn't received a lot of investigation. Greetings, Andres Freund -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
