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VMware Multi-Writer Attribute for Shared VMDKs

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:

  • Because vSAN does not support raw device mappings (RDMs), this document applies only to virtual disks resident on the vSAN datastore.
  • Because vSAN does not support SCSI-3 persistent reservations, storage-based fencing is not supported using multiwriter mode. Host-based fencing must be used instead.
  • When using the multiwriter mode, the virtual disk must be eager-zeroed thick (EZT). When creating an EZT disk on vSAN, the disk is not zeroed automatically. If zeroing is required, use the vmkfstools -w command to zero-out all blocks. For more information, see VMware knowledge base article 1033570, “Powering on the virtual machine fails with the error: Thin/TBZ disks cannot be opened in multiwriter mode.”
  • Sharing virtual disks in multiwriter mode on vSAN is limited to 8 ESXi/ESX 6.x hosts.
  • Hot-adding a virtual disk removes the multiwriter flag. For more information, see VMware knowledge base article 2078540, “Hot adding a virtual disk in ESXi 5.5 removes the multiwriter flag.”
  • VMs with multiwriter VMDKs cannot be protected with VMware Site Recovery Manager™ or VMware vSphere Replication™.

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.