oracleasm createdisk failed
Solution:   The ASM error "disk does not exist or is not instantiated" error can be caused by many issues:
     
     - Not disabling SELINUX:
        vi /etc/sysconfig/selinux SELINUX=disabled
        Then reboot your system and run getenforce to confirm that 
        SELINUX is disabled: 
        root> /usr/sbin/getenforce Disabled 
Commentaires
http://www.dba-oracle.com/t_asm_disk_does_not_exist_or_is_not_instantiated.htm
La solution se trouve ici :
https://docs.oracle.com/cd/E52668_01/E60259/html/section_ocp_gvd_tq.html
The oracleasm script fails if SELinux is in Enforcing mode. The suggested workaround is to disable the SELInux policy module for Oracle ASM before running oracleasm:
# semodule -d oracleasm
# semodule -l | grep oracleasm
oracleasm 1.0.0 Disabled