Knowledge Base
Disabling simultaneous write protection provided by VMFS using the multi-writer flag (1034165)
Details
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.
It is possible to 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 will not cause data loss. This document describes the steps that need to be taken to modify the flag for a virtual disk.
Caution: 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. Specifically supported virtual machine operations include: power on/off (but not suspend), restart, hot-add disks to existing adapaters, hot-remove devices, connect/disconnect devices. The virtual machine will run its operating system and applications as expected. See a list of limitations 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 where the applications ensure that writes originating from two or more different virtual machines will 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.
- When using the multi-writer mode, 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).
Note: This does not conflict with VMware's recommendation to use eager zeroed thick virtual disks when deploying Oracle databases.
- Sharing is limited to eight ESX hosts.
- Linked clones and snapshots are not supported. Be aware that other vSphere activities utilize cloning and that backup solutions leverage snapshotting via the vAPIs.
- CBT with multi-writer is not supported.
- Storage vMotion of a virtual machine with any of it's disks configured with the multi-writer flag is not supported.
- Suspending a virtual machine is not supported.
- You cannot hot-extend a disk.
Solution
Enabling Virtual Disk Sharing
- Create a virtual disk to be shared using any acceptable approach. For example, through the vSphere Client:
- Create a new disk for a virtual machine on a VMFS data store:
- Select Support clustering features such as Fault Tolerance. This creates the new virtual disk in the required format, eager zeroed thick.
Note: ESXi 5.0 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.
- Choose the appropriate Virtual Device Node setting, such as SCSI (1:0). Make a note of it, as it is required later.
Note: There are command-line alternatives to perform the previous operation, but this instruction is beyond the scope of this article.
- Create a new disk for a virtual machine on a VMFS data store:
- Add the multi-writer flag to configure sharing. There are two ways to achieve this:
Note: Do not change the SCSI Controller's SCSI Bus Sharing options. Keep the default selection, None.
- Power off the virtual machine.
- 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"
- Save the .vmx file and power on the virtual machine.
- In the vSphere Client, power off the virtual machine, go to Edit Settings > Options > Advanced > General > Configuration Parameters. Add rows for each of the shared disks and set their value to multi-writer.
Example:
- Add this disk to another virtual machine:
- In the vSphere Client inventory, right-click the virtual machine and click Edit Settings.
- Click the Hardware tab and click Add.
- Select Hard Disk and click Next.
- Select Use an Existing Virtual Disk.
Note: You do not need to create a disk because the disk is already created.
- In the vSphere Client inventory, right-click the virtual machine and click Edit Settings.
- Enable sharing on this virtual machine, using one of the approaches listed in step 2.
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