VMware virtual disk (VMDK) in Multi Write Mode

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. This is the safety mechanism to avoid data corruption in cases where the applications in the virtual machine do not maintain consistency in the writes performed to the shared disk. However, you might have some third-party cluster-aware application, where the multi-writer option allows VMFS-backed disks to be shared by multiple virtual machines and leverage third-party OS/App cluster solutions to share a single VMDK disk on VMFS filesystem. These third-party cluster-aware applications, in which the applications ensure that writes originate from multiple different virtual machines, does not cause data loss. Examples of such third-party cluster-aware applications are Oracle RAC, Veritas Cluster Filesystem, etc.

There is VMware KB “Enabling or disabling simultaneous write protection provided by VMFS using the multi-writer flag (1034165)” available at https://kb.vmware.com/kb/1034165 KB describes how to enable or disable simultaneous write protection provided by VMFS using the multi-writer flag. It is the official resource how to use multi-write flag but the operational procedure is a little bit obsolete as vSphere 6.x supports configuration from WebClient (Flash) or vSphere Client (HTML5) GUI as highlighted in the screenshot below.

However, KB 1034165 contains several important limitations which should be considered and addressed in solution design. Limitations of multi-writer mode are:

• The virtual disk must be eager zeroed thick; it cannot be zeroed thick or thin provisioned.
• Sharing is limited to 8 ESXi/ESX hosts with VMFS-3 (vSphere 4.x) and VMFS-5 (vSphere 5.x) and VMFS-6 in multi-writer mode.
• Hot adding a virtual disk removes Multi-Writer Flag. 

Let’s focus on 8 ESXi host limit. The above statement about scalability is a little bit unclear. That’s the reason why one of my customers has asked me what does it really mean. I did some research on internal VMware resources and fortunately enough I’ve found internal VMware discussion about this topic, so I think sharing the info about this topic will help to broader VMware community.

Here is 8 host limit explanation in other words …

“8 host limit implies how many ESXi hosts can simultaneously open the same virtual disk (aka VMDK file). If the cluster-aware application is not going to have more than 8 nodes, it works and it is supported. This limitation applies to a group of VMs sharing the same VMDK file for a particular instance of the cluster-aware application. In case, you need to consolidate multiple application clusters into a single vSphere cluster, you can safely do it and app nodes from one app cluster instance can run on other ESXi nodes than app nodes from another app cluster instance. It means that if you have more than one app cluster instance, all app cluster instances can leverage resources from more than 8 ESXi hosts in vSphere Cluster.”

   

The best way to fully understand specific behavior is to test it. That’s why I have a pretty decent home lab. However, I do not have 10 physical ESXi host, therefore I have created a nested vSphere environment with vSphere Cluster having 9 ESXi hosts. You can see vSphere cluster with two App Cluster Instances (App1, App2) on the screenshot below.

Application Cluster instance App1 is composed of 9 nodes (9 VMs) and App2 instance just from 2 nodes. Each instance is sharing their own VMDK disk. The whole test infrastructure is conceptually depicted on the figures below.

Test Step 1: I have started 8 of 9 VMs of App1 cluster instance on 8 ESXi hosts (ESXi01-ESXi08). Such setup works perfectly fine as there is 1 to 1 mapping between VMs and ESX hosts within the limit of 8 ESXi hosts having shared VMDK1 opened.

Test Step 2: Next step is to test the Power-On operation of App1-VM9 on ESXi09. Such operation fails. This is expected result because 9^th ESXi host cannot open the VMDK1 file on VMFS datastore.

The error message is visible on the screenshot below.

Test Step 3: Next step is to Power On App1-VM9 on ESXi01. This operation is successful as two app cluster nodes (virtual machines App1-VM1 and App1-VM9) are running on single ESXi host (ESX01) therefore only 8 ESXi hosts have the VMDK1 file open and we are in the supported limits.

Test Step 4: Let’s test vMotion of App1-VM9 from ESXi01 to ESX09. Such operation fails. This is expected result because of the same reason as on Power-On operation. App1 Cluster instance would be stretched across 9 ESXi hosts but 9^th ESXi host cannot open VMDK1 file on VMFS datastore.

The error message is a little bit different but the root cause is the same.

Test Step 5: Let’s test vMotion of App2-VM2 from ESXi08 to ESX09. Such operation works because App2 Cluster instance is still stretched across two ESXi hosts only so it is within supported 8 ESXi hosts limit.

Test step 6: The last test is the vMotion of App2-VM2 from vSphere Cluster (ESXi08) to standalone ESXi host outside of the vSphere cluster (ESX01). Such operation works because App2 Cluster instance is still stretched across two ESXi hosts only so it is within supported 8 ESXi hosts limit. vSphere cluster is not the boundary for multi-write VMDK mode.

FAQ

Q: What exactly does it mean the limitation of 8 ESXi hosts?

A: 8 ESXi host limit implies how many ESXi hosts can simultaneously open the same virtual disk (aka VMDK file). If the cluster-aware application is not going to have more than 8 nodes, it works and it is supported. Details and various scenarios are described in this article.

Q: Where are stored the information about the locks from ESXi hosts?

A: The normal VMFS file locking mechanism is in use, therefore there are VMFS file locks which can be displayed by ESXi command: vmkfstools -D

The only difference is that multi-write VMDKs can have multiple locks as is shown in the screenshot below.

Q: Is it supported to use DRS rules for vmdk multi-write in case that is more than 8 ESXi hosts in the cluster where VMs with configured multi-write vmdks are running?

A: Yes. It is supported. DRS rules can be beneficial to keep all nodes of the particular App Cluster Instance on specified ESXi hosts. This is not necessary nor required from the technical point of view, but it can be beneficial from a licensing point of view.  

Q: How ESXi life cycle can be handled with the limit 8 ESXi hosts?

A: Let’s discuss specific VM operations and supportability of multi-write vmdk configuration. The source for the answers is VMware KB https://kb.vmware.com/kb/1034165

·      Power on, off, restart virtual machine – supported

·      Suspend VM – unsupported

·      Hot add virtual disks - only to existing adapters

·      Hot remove devices – supported

·      Hot extend virtual disk – unsupported

·      Connect and disconnect devices – supported

·      Snapshots – unsupported

·      Snapshots of VMs with independent-persistent disks – supported

·      Cloning – unsupported

·      Storage vMotion – unsupported

·      Changed Block Tracking (CBT) – unsupported

·      vSphere Flash Read Cache (vFRC) – unsupported

·      vMotion – supported by VMware for Oracle RAC only and limited to 8 ESX/ESXi hosts. Note: other cluster-aware applications are not supported by VMware but can be supported by partners. For example, Veritas products have supportability documented here https://sort.veritas.com/public/documents/sfha/6.2/vmwareesx/productguides/html/sfhas_virtualization/ch01s05s01.htm Please, verify current supportability directly with specific partners.

Q: Is it possible to migrate VMs with multi-write vmdks to different cluster when it will be offline?

A: Yes. VM can be Shut Down or Power Off and Power On on any ESXi host outside of the vSphere cluster. The only requirement is to have the same VMFS datastore available on source and target ESXi host. Please, keep in mind that the maximum supported number of ESXi hosts connected to a single VMFS datastore is 64.

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UPDATE 2019-07-10: From vSphere 6.7 Update 1 onwards, the virtual disks sharing support in multi-writer has been extended to more than 8 hosts. In order to enable this feature, you need to enable /VMFS3/GBLAllowMW advance config option. For more info see https://kb.vmware.com/s/article/1034165