The documentation seems to say that when setting up a standby, it is OK to rsync/copy the files from the master, but the pg_xlog directory should be excluded. Is there some important difference between the files in pg_xlog in the master and those in the slave? If so, what is it? If the pg_xlog files are copied, would it cause the "timeline xx of the primary does not match recovery target timeline yy" error? Why?