ASM disk

A disk is a primary element of an ASM instance. A disk could be built or formed either from a Logical Unit Number (LUN) by a storage array, a disk partition or an entire disk, or a logical volume or Network Attached File (NFS). An ASM instance discovers the disks within the paths specified with the ASM_DISKSTRING initialization parameter. Whenever a new disk is discovered by the ASM instance, the header status of the disk is set to the CANDIDATE flag and makes the disk available in the instance.
After the disks are successfully discovered by the local ASM instance, they will appear in the V$ASM_DISK dynamic view of the local ASM instance and the disks are ready to use to build a new ASM disk group, or can be added to any pre-existing
ASM disk groups. However, when no particular paths for the disks are specified with the ASM_DISKSTRING initialization parameter, ASM by default look in the following OS specific paths to discover the disks:



On a cluster environment, ensure that all ASM disks are visible across all nodes and each node must have the exact set of permissions (660) and ownership (oracle:dba) to avoid running into any sorts of problems. Oracle also strongly advises to have the
same size of disks in a disk group to maintain the disk group balance. On the other hand, you have the flexibility to define a different naming convention for a disk across the nodes in a cluster.

The following limits, as of 11g R2, have been imposed on ASM disks:
• A maximum of 10,000 disks
• Up to 2 TB maximum storage per ASM disk, with EXADATA 4PB per ASM disk