File removed while backup is running

Hi,

I have two postgres cluster in 9.4 version and all run backups correctly.
Recently I add a new cluster and the backup doesn't work on it.

All databases have archive_mode = on and wal_level=archive

My backup has this steeps:

SELECT pg_start_backup('${clustername}');"
tar PGDATA and TABLESPACES
Select pg_stop_backup();
tar WAL files

With this database when I run backup I get errors like:

t3_28961_fsm: File removed before we read it

I don't know what file type is t3_28961_fsm.

If I go to tablespace dir i have several t files and this file types are
adding and removing continously.

-rw------- 1 postgres postgres 8.0K May 21 09:01 t3_29094
-rw------- 1 postgres postgres 0 May 21 09:01 t3_29087
-rw------- 1 postgres postgres 8.0K May 21 09:01 t3_29093

I'm looking for this error but i didn't found any.

Any idea?

Thanks in advance.

Regards.

Mario