wired problem for a 9.1 slave：receive wal but do not replay it？

Today,our monitor report a pg slave instance'disk space usage reach 96%，I
login in to the machine，and find the pg_xlog dir take up more than
2TB，which is abnormal.
the number of WAL file in the pg_xlog dir is more than 130k,while we set
the wal keep number to 8192.
I think there is something stop the replay,so I check the
pg_stat_activity,pg_prepare_statement,pg_xact etc,but find all normal.
I run：
ps auxwww | grep postgres
and can find the wal receiver and streaming receiver work happily，because
the wal file name，the streaming log id changed.

So I have no idea.

I then restart the slave PG,and find it recover from a very old wal which
is one month ago.
We are now set up a new slave for the master while let the recover from
this slave go.

the PG version is 9.1.9,OS is CentOS 6 x86-64.

Jov
blog: http:amutu.com/blog <http://amutu.com/blog>