VMFS is a clustered file system that disables (by default) multiple virtual machines from opening and writing to the same virtual disk (.vmdk file). This prevents more than one virtual machine from inadvertently accessing the same .vmdk file.

The multi-writer option allows VMFS-backed disks to be shared by multiple virtual machines. This option is used to support VMware fault tolerance, which allows a primary virtual machine and a standby virtual machine to simultaneously access a .vmdk file.

You can use this option to disable the protection for certain cluster-aware applications where the applications ensure that writes originating from two or more different virtual machines does not cause data loss. This document describes methods to set the multi-writer flag for a virtual disk.

Warning: Be careful when you disable this protection. This change might cause data corruption in cases where the applications in the virtual machine do not maintain consistency in the writes performed to the shared disk. As a result, some virtual machine operations and vSphere features are not supported and these are listed below.

Use cases:

• VMware FT relies on VMware vLockstep technology to establish and maintain an active secondary virtual machine that runs in virtual lockstep with the primary virtual machine. The secondary virtual machine resides on a different host and executes exactly the same sequence of virtual (guest) instructions as the primary virtual machine. The multi-writer flag is automatically enabled when you turn on FT in the vSphere Client.

• Third-party cluster-aware applications, in which the applications ensure that writes originate from two or more different virtual machines, does not cause data loss (for example, you would use this for Oracle RAC virtual machines). For this case, manually enable the multi-writer flag using the instructions in this article. Follow the same procedure when using virtual RDMs.

• Starting vSphere 7.0 Update 1, NVMe over Fibre Channel and NVMe over RDMA (RoCE v2) datastores now support multi-writer option enabling in-guest systems that leverage cluster-aware file systems to have distributed write capability. Please refer to the VMware NVMe Storage guidelines before configuring the NVMe Storage for shared disks with the multi-writer attribute. For more information, see About VMware NVMe Storage

• Starting vSphere 7.0 Update 3, vVOL datastore backed by FC, iSCSI protocols now support multi-writer option enabling in-guest systems that leverage cluster-aware file systems to have distributed write capability. For more information, see Oracle workloads on VMware Virtual Volumes (vVOLS) using Pure Storage FlashArray X50 and Broadcom LPe36000 Fibre Channel Adapter – better performance

• Starting vSphere 8.0 Update 2, vVOL datastores backed by NVMe FC, NVMe TCP  also supports multi-writer option .

• For Oracle RAC virtual machines to work correctly with the multi-writer flag enabled the ESXi 5.5 hosts the virtual machines are running on must be patched to VMware ESXi 5.5, Patch ESXi550-201407401-BG: Updates esx-base (2077407) or later.

• For configurations using VMware vSAN, see Using Oracle RAC on vSphere 6.x vSAN Datastore (2121181).

• Support information for Veritas InfoScale in ESXi guests can be found at Veritas InfoScale Enterprise 7.x in guest storage, high availability, and disaster recovery support (Partner Verified and Supported) (51476)

• When using the multi-writer mode for disks from VMFS datastore, the virtual disk must be eager zeroed thick; it cannot be zeroed thick or thin provisioned. For more information, see A virtual machine fails to power on with the error: Thin/TBZ disks cannot be opened in multiwriter mode. VMware ESX cannot open the virtual disk for clustering. (1033570).

• When using the multi-writer mode for disks from vVOLs datastore, the virtual disk can be either thin or thick.

• Hot adding a virtual disk removes Multi-Writer Flag. For more information, see Hot adding a virtual disk in ESXi 5.5 removes the multi-writer flag (2078540).

• From vSphere 8.0 update 2 onwards virtual NVMe controller is  also supported for the virtual disks with multi-writer mode.

• From vSphere 6.7 Update 1 onwards, the virtual disks sharing support in multi-writer has been extended to more than 8 hosts. "This applies to VMFS5 / VMFS6 as well". In order to enable this feature, you need to enable /VMFS3/GBLAllowMW advance config option  as shown below:

1. Login to ESXi host and execute the below command:

2.  Execute the below command to check the value set.

3.  Enable the above config option on ALL the hosts that are part of the shared environment.

Enabling Virtual Disk Sharing

1. Create a virtual disk to be shared using any acceptable approach. For example, through the vSphere Client:
   1. Create a new disk for a virtual machine on a VMFS data store:
      
      
       
   2. In the Create a Disk window, for ESX/ESXi versions earlier than 5.0, click Support clustering features such as Fault Tolerance to create the new virtual disk in the required format, eager zeroed thick.
      
      
      
      In ESXi 5.x versions, the Create a Disk window provides Thick Provision Lazy Zeroed, Thick Provision Eager Zeroed, and Thin Provision options under Disk Provisioning. Be sure to select Thick Provision Eager Zeroed. Also, note that the documentation may refer to this as Flat pre-initialized.
      
      
       
   3. Choose a Virtual Device Node setting, such as SCSI (1:0). Make a note of it, as it is required later.
      
      
      
       
2. Add the multi-writer flag to configure sharing.
   
   Note: Do not change the SCSI Controller's SCSI Bus Sharing options. Keep the default selection, None.
   
   To enable Multi Writer flag for sharing a particular disks, apply one of these options:
   1.  
      1. Power off the virtual machine.
      2. In the . vmx file that defines the virtual machine, add an entry similar to:
         scsiX:Y.sharing = "multi-writer"
         
         where X is the controller ID and Y is the disk ID on that controller. The setting screen of a virtual machine shows these values.
         
         
         
         Add this setting for each virtual disk that you want to share. For example, to share four disks, the configuration file entries look like this:
         scsi1:0.sharing = "multi-writer"
         scsi1:1.sharing = "multi-writer"
         scsi1:2.sharing = "multi-writer"
         scsi1:3.sharing = "multi-writer"
          
      3. Save the .vmx file and power on the virtual machine.
         
         Note: In vSphere 6.0 and later, the GUI has the capability of setting Multi Writer flag without any need to edit .vmx file.
          
   2. In the vSphere Client, power off the virtual machine, navigate to Edit Settings > Options > Advanced > General > Configuration Parameters. Add rows for each of the shared disks and set their values to multi-writer.
      
      
       
3. Add this disk to another virtual machine:
    
   1. In the vSphere Client inventory, right-click the virtual machine and click Edit Settings.
   2. Click the Hardware and click Add.
   3. Select Hard Disk and click Next.
   4. Select Use an Existing Virtual Disk.
      
      Note: You do not need to create a disk because the disk is already created.
       
4. Enable sharing on this virtual machine, using one of the approaches listed in step 2.

Additional Information

• Español: Deshabilitar la protección de escritura simultánea que brinda VMFS con el indicador de escritura múltiple (2072887)
• Português: Como desabilitar a proteção contra gravação simultânea fornecida pelo VMFS usando o sinalizador multi-writer (2046562)
• 日本語: マルチライター フラグを使用して、VMFS が提供する同時書き込み保護を無効にする (2080040)
• 简体中文: 使用多写入器标记禁用 VMFS 提供的同步写入保护 (2076182)