do the files in pg_xlog differ in master and slave?

Hi,

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?

Thanks!

--
Kent Tong
IT author and consultant, child educator