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.” and  VMware knowledge base article 1034165 ,  "Enabling or disabling simultaneous write protection provided by VMFS using the multi-writer flag (1034165)".

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.

More information on restrictions to using shared disks with the multiwriter attribute on VMware vSAN can be found in VMware knowledge base article 2121181 .

Important points to be kept in mind :

  • Shared VMDKs need not be set to independent persistent mode for the multiwriter attribute
  • Prior to vSAN 6.7 Patch 01, for multi-writer mode to be enabled, the shared VMDKs must be Eager Zero Thick provisioned (EZT) (OSR=100)
  • Starting VMware vSAN 6.7 Patch 01, Oracle RAC on vSAN does NOT require the shared VMDKs to be Eager Zero Thick provisioned (OSR=100) for multi-writer mode to be enabled. Shared VMDKs can be thin provisioned (OSR=0) for multi-writer mode to be enabled. For existing Oracle RAC deployments migrating to this vSAN 6.7 Patch 01 vSAN version or higher , one can use SPBM (Storage Policy Based management) to change the existing storage policy from OSR = 100 to OSR 0 ( Or vice versa if required). This is an online change and does not require a downtime
  • Starting vSAN 6.2, the Client Cache , used on both hybrid and all flash vSAN configurations, leverages DRAM memory local to the virtual machine to accelerate read performance. The amount of memory allocated is 0.4% up to 1GB per host. As the cache is local to the virtual machine, it can properly leverage the latency of memory by avoiding having to reach out across the network for the data. In testing of read cache friendly workloads it was able to significantly reduce read latency. For more information on Client Cache feature, see VMware vSAN Design and Sizing Guide. The only caveat is Client Cache will be disabled automatically for a vmdk if the Multi-Writer is enabled.  Enabling multi-writer indicates that that the vmdk can be opened from multiple VMs from same or different hosts. The Client cache being per-host local basis , its is used for targeting re-use of the local hot data for a particular host. For this reason, Client cache is disable for vmdk’s shared with multi-writer attribute

For VMware Cloud on AWS Prior SDDC Version 1.8v2 , All RAC shared VMDKs set to EZT, a requirement for turning on the multiwriter attribute. Starting VMware Cloud on AWS SDDC version v1.8v2 , vSAN datastores in VMware Cloud on AWS can now support multi-writer mode on Thin-Provisioned VMDKs  Find here more information on the article which details Oracle RAC with Thin provisioned shared vmdk’s on VMware Cloud on AWS starting 1.8v2

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.

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