A new file system is mounted to a mount point of an existing file system implemented in a computational system. A sentinel indicator is stored in the mount point. Prior to initiating an input/output (I/O) operation by a program, a determination is made by the program as to whether the sentinel indicator exists. In response to determining by the program that the sentinel indicator exists, the I/O operation is performed by the program. In certain alternative embodiments, in response to determining by the program that the sentinel indicator exists, the I/O operation is not performed by the program.