In previous release, ASM clients can only access ASM using ASM instance running on the same server. If ASM instance on that node fails, the clients all fail. With Flex ASM removes the hard dependency between ASM and database clients and therefore it does not require ASM instance on each node. ASM clients can use a remote ASM network for communication (ASM metadata, data blocks, and etc.) between ASM and its clients network connection to access ASM, so if ASM instance fails, its clients can connect to another instance. The default cardinality for ASM instances is three (similar as scan) regardless of the cluster size. Its different with scan is that with two nodes flex cluster, there are 2 ASM instances not 3.
Up to three ASM listeners are registered as remote listeners for each database client. When setting up 12c, if you choose to configure a flex Cluster, Flex ASM is inherited and you must specify an ASM network. However, ASM does not require a Flex cluster. It can run on standard cluster providing I/O services and there are no new instance parameters for Flex ASM. The default parameter settings are suitable to support most situations.
Clients are automatically relocated to another instance and are transparent to end users. DBAs can query v$asm_client and manual relocation using the command “alter system relocate client <client_id>) before planned maintenance, applying a patch or adjust the workload balance between instances.
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