Scott Briggs wrote: > Hi, can someone please explain the purpose of archive_command on both > the master and slave when it comes to streaming replication? From > what I understand so far, what really matters is how many pg_xlog > files are kept when it comes to reestablishing replication when it > breaks for some reason. > > Let's say I shutdown one slave to create a second slave by copying all > the files to that new slave. If I want to guarantee that the original > slave is able to reestablish replication, I just need to make sure > that the master keeps enough pg_xlog files (defined by the setting > "checkpoint_segments") around in order for that to happen (especially > in the event that the database is large).
First, there are other uses of archive_command than streaming replication - you may want to recover the database in case of problems. Also, it is a good idea to have the setting the same (or comparable) on master and slave so that promotion of the slave to master is easier. But in principle you are right; if you keep the WAL files in pg_xlog long enough, you won't need the archives for streaming replication. Note, however, that the parameter you must set to retain them long enough is wal_keep_segments and not checkpoint_segments. I think that the main reason to have shared WAL archives in streaming replication is that it still may happen that the slave lags too long, like a server crash or a network problem. That could then mean that you have to rebuild the slave if you have no WAL archives that enable the slave to catch up. Yours, Laurenz Albe -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
