I've found out that archive_command is only run by postgresql on a master.
If I'm running a cascading replication like:
Master -> Slave1 -> Slave2 and want to archive WAL segments on Slave1, the command is never run.
Is this correct?
If so, what if I just copy (rsync or similar) myself the WAL segments from pg_xlog directory to my archive directory? Could I then use this archive directory as the source for a restore_command in a new slave?