As occurs with VMware vSphere VMFS and NFS datastores, vSAN prevents multiple virtual machines (VMs) from opening the same virtual machine disk (VMDK) in read/write mode. This protects the data stored on the virtual disk from corruption caused by multiple writers on the non–cluster-aware file systems used by most guest OSs.
To enable in-guest systems that leverage cluster-aware file systems that have distributed write (multiwriter) capability, we must explicitly enable multiwriter support for all applicable VMs and VMDKs.
vSAN also supports the multiwriter option to enable in-guest systems that leverage cluster-aware file systems that have distributed write capability.
A list of supported and unsupported actions associated with enabling the multiwriter attribute can be found in VMware knowledge base article 2121181, “Using Oracle RAC on a vSphere 6.x vSAN Datastore.”
Table 1 shows supported and unsupported actions or features with multiwriter flag.
NOTE: vSphere Storage vMotion is unsupported for shared disks using the multiwriter attribute.
VMware knowledge base article 2121181 cites the following restrictions to using shared disks with the multiwriter attribute:
NOTE: Shared VMDKs need not be set to independent persistent mode for the multiwriter attribute.
Below shows an example of how the multiwriter flag for a shared VMDK can be set using the vSphere Web Client for VMDKs backed by VMware hyper-converged infrastructure (HCI) vSAN.
Find here the steps for adding shared VMDKs using the multiwriter attribute to an Oracle RAC online without any downtime.
The storage option for Oracle workload has been discussed in detail here.