How could we start Oracle database in Oracle Exadata which's celldisk has proactive failure?
Exadata uses NORMAL or HIGH redundancy diskgroups. It is possible to use EXTERNAL redundancy diskgroups as well, but that is not supported.

Even if you drop a griddisk, and one of the ASM failgroups becomes incomplete, you can still mount the ASM diskgroup and use the mirror side(s). But this will not happen automatically, because the usual MOUNT command throws an error in such a case, and srvctl will not start the database because of that.

You should be able to mount the diskgroup manually with the FORCE option. Log in to the ASM instance as sysasm:

sqlplus / as sysasm


And mount the diskgroup (on all nodes):

alter diskgroup data_dm01 mount force;


Once the diskgroup is mounted, you should be able to start the database.

For the faulty physicaldisk, you should open an SR with Oracle (given that you have valid support contract) for a replacement drive (this usually automatically happens with ASR (Auto Service Request)). The last time I did that, the replacement drive was available in 1-2 days, and their engineer delivered and replaced it in the customers data center.