The value for the ASM_DISKSTRING initialization parameter is an operating system–dependent value that Oracle ASM uses to limit the set of paths that the discovery process uses to search for disks. When a new disk is added to a disk group, each Oracle ASM instance that has the disk group mounted must be able to discover the new disk using its ASM_DISKSTRING.
In many cases, the default value (NULL) is sufficient. Using a more restrictive value might reduce the time required for Oracle ASM to perform discovery, and thus improve disk group mount time or the time for adding a disk to a disk group. Oracle may dynamically change the ASM_DISKSTRING before adding a disk so that the new disk is discovered through this parameter.
The default value of ASM_DISKSTRING might not find all disks in all situations. If your site is using Oracle ASM Filter Driver or a third-party vendor, then you may have discovery string conventions that you must use for ASM_DISKSTRING. For information about Oracle ASM Filter Driver, refer to "Oracle ASM Filter Driver".
In addition, if your installation uses multipathing software, then the software might place pseudo-devices in a path that is different from the operating system default. See "Oracle ASM and Multipathing" and consult the multipathing vendor documentation for details.