ORA-15027: active use of diskgroup "string" precludes its dismount
Cause: An ALTER DISKGROUP ... DISMOUNT command specified a diskgroup which had database client instances with open files in the diskgroup. Diskgroups cannot be dismounted until all open files in the diskgroup are closed by the database client instances.
Action: Stop all clients that are using this diskgroup and retry the ALTER DISKGROUP ... DISMOUNT command. The V$ASM_CLIENT fixed view in an ASM instance provides a list of its active database client instances.
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ORA-15027active use of diskgroup "string" precludes its dismount