The ASM_DISKSTRING initialization parameter specifies a comma-delimited list of strings that limits the set of disks that an Oracle ASM instance discovers. The discovery strings can include wildcard characters. Only disks that match one of the strings are discovered. The same disk cannot be discovered twice.
The discovery string format depends on the Oracle ASM library and the operating system that are in use. Pattern matching is supported. Refer to your operating system-specific installation guide for information about the default pattern matching.
For example on a Linux server, to limit the discovery process to only include disks that are in the /dev/rdsk/mydisks directory for an Oracle ASM instance that does not use Oracle ASM Filter Driver (Oracle ASMFD) or ASMLIB, set the ASM_DISKSTRING initialization parameter to:
/dev/rdsk/mydisks/*
The asterisk is required.
To limit the discovery process to only include disks that have a name that ends in disk3 or disk4, you could set ASM_DISKSTRING as follows on a Linux system:
ASM_DISKSTRING = '/dev/rdsk/*disk3', '/dev/rdsk/*disk4'
The ? character, when used as the first character of a path, expands to the Oracle home directory. Depending on the operating system, when you use the ? character elsewhere in the path, it is a wildcard for one character.
The default value of the ASM_DISKSTRING parameter is a NULL string. A NULL value causes Oracle ASM to search a default path for all disks in the system to which the Oracle ASM instance has read and write access. The default search path is platform-specific. Refer to your operating system-specific installation guide for more information about the default search path.
Oracle ASM cannot use a disk unless all of the Oracle ASM instances in the cluster can discover the disk through one of their own discovery strings. The names do not have to be the same on every node, but all disks must be discoverable by all of the nodes in the cluster. This may require dynamically changing the initialization parameter to enable adding new storage.
For additional information about discovering disks, see "Oracle ASM Disk Discovery ".
Oracle Exadata documentation for information about the Oracle ASM discovery string format for Oracle Exadata
Oracle Database Reference for more information about the ASM_DISKSTRING initialization parameter