ASM Proxy: New Instance Type in Oracle 12c

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Prior to Oracle Database 12c, an ASM instance ran on every node in the cluster and ASM Cluster File System (ACFS) Service on a node connected to the local ASM instance running on the same host to fetch the required metadata. If the ASM instance on a node were to fail, then ACFS file systems could no longer be accessed on that node.

With introduction of Flex ASM in Oracle 12c, hard dependency between ASM and its clients has been relaxed and only a smaller number of ASM instances need run on a subset of servers in a cluster. In such a scenario, in order to make ACFS services available on nodes without an ASM instance, a new instance type has been introduced by Flex ASM: the ASM-proxy instance which works on behalf of a real ASM instance. ASM Proxy instance fetches the metadata about ACFS volumes and file systems from an ASM instance and caches it. If ASM instance is not available locally, ASM proxy instance connects to other ASM instances over the network to fetch the metadata. Additionally, if the local ASM instance fails, then ASM proxy instance can failover to another surviving ASM instance on a different server resulting in uninterrupted availability of shared storage and ACFS file systems.

Whenever IO needs to be performed on an ACFS, the ASM-Proxy instance passes on the extent map and disk list information to the ADVM driver. Subsequently, this metadata is cached by the ADVM driver. ADVM directs all the ACFS IOs to a specific ASM disk group disk (disks) location, including any mirrored extent updates. In other words, all ACFS IOs are written through the ADVM OS Kernel driver directly to storage. No IOs are delivered through the ASM proxy or ASM instance.

Which nodes can host ASM-Proxy instance?

ASM-Proxy instance only needs to run in the clusters employing Flex ASM on the nodes where access to ACFS is required. Whereas ASM-Proxy instance can run on any node in a standard cluster, only Hub nodes in a Flex cluster can host it (Fig. 1). It can run on the same node as ASM instance or on a different node. It can be shut down when ACFS is not running.
Which clusters use the ASM proxy instance

Fig. 1

The ASM Proxy instance has:

  • INSTANCE_TYPE initialization parameter set to ASMPROXY
  • ORACLE_SID set to +APX<node number>.

In this article, I will demonstrate that with Flex ASM:

  • Metadata related to ACFS is cached in ASM-Proxy instance rather than ASM instance
  • ASM-Proxy instance obtains the metadata related to ACFS from:
    • An ASM instance running locally
    • An ASM instance running remotely if the local ASM instance is not running
  • Availability of ACFS on a node:
    • Requires that ASM proxy instance must be running on that node
    • Is not affected by the failure of the local ASM instance

For this demonstration, I have set up a two node 12.1.0.2c Flex Cluster (with Flex ASM).

Overview:

  • Create a Cloud File System Resource on DATA disk group.
  • Verify that:
    • All the ASM and ACFS-related resources are running on both the nodes
    • Metadata related to ACFS is cached in ASM-Proxy instance rather than ASM instance
    • ASM-Proxy instance obtains the metadata related to ACFS from ASM instance running locally
  • Verify that on stopping ASM on a node (host02):
    • The ASM-Proxy instance obtains the metadata related to ACFS from an ASM instance running remotely
    • Availability of ACFS on the node is not affected
  • Verify that on stopping ASM- Proxy instance on a node (host02), ACFS cannot be accessed on that node even if ASM instance is available.

Demonstration:

  • Create a Cloud File System Resource on DATA disk group:
    • Check that Proxy instance is not running presently

  • Create a volume VOL1 on DATA disk group

    • As soon as the volume is created, an ASM proxy instance is automatically started on both nodes.

    • Create an ACFS File System on the newly created volume VOL1

    • Create Corresponding Mount Points on both nodes

    • Configure and start Cloud File System Resource on the volume device VOL1 with the mount point /mnt/acfsmounts/acfs1

    • Verify the Cloud File System Resource by creating a small text file created on it from host01 and then accessing it successfully from host02

  • Verify that All the ASM and ACFS-related resources are running on both nodes

  • Verify that Metadata for ACFS is cached in not cached in ASM instance

  • Verify that Metadata for ACFS is cached in ASM-Proxy instance:

  • Verify that both the ASM-Proxy instances obtain the metadata related to ACFS from ASM instance running locally:

  • Verify that on stopping ASM on a node (host02),
    • ASM-Proxy instance obtains the metadata related to ACFS from an ASM instance running remotely
    • Availability of ACFS on the node is not affected

  • Verify that on stopping ASM- Proxy instance on a node (host02), ACFS cannot be accessed on that node even though ASM instance is present:

Thus in a flex cluster, all metadata about ACFS is cached in ASM-Proxy instance. Also, availability of ACFS Volume and ACFS on a node is dependent on availability of an ASM–Proxy instance only on the node irrespective of the status of local ASM instance.

Summary

  • In a cluster with an ASM instance running on every node:
    • Metadata related to ACFS is cached in the ASM instance
    • Failure of the local ASM instance disrupts the availability of ACFS on that node
  • A new instance type has been introduced by Flex ASM – the ASM proxy instance, which gets metadata information from ASM instance on behalf of ACFS.
  • In a cluster with flex ASM
    • ASM instances run on a subset of nodes
    • Metadata related to ACFS is cached in the ASM-Proxy instance rather than the ASM instance
    • The ASM-Proxy instance obtains the metadata related to ACFS from the ASM instance running locally / remotely
    • Availability of ACFS on a node is dependent on availability of ASM–Proxy instance only irrespective of the status of local ASM instance.