This can only be safely used if you have a “cluster-aware” application that understands how to coordinate disk I/O activity from more than one VM to the same VMDK without causing corruption. However, the multi-writer flag is a VMFS option that turns on the functionality for more than one VM to write to a single VMDK. Typically, bad things can start to happen if two VMs try to write to the same VMDK in an uncoordinated way. When it comes to a single VMDK virtual disk, VMFS has the multi-writer flag disabled by default. This means it understands multiple hosts talking to the same storage subsystem. VMware vSphere uses the VMFS file systems which is a “cluster-aware” file system. To give just a bit of background on the multi-writer flag and what it technically achieves, let’s get a bit of background on the technology. What is VMware vSphere Multi-writer flag? What about backing up virtual machines that are configured with a multi-writer disk? How can you successfully protect these types of workloads? Let’s take a look at how to backup virtual machines with shared multi-writer disks. You can take a look at my post here of how to provision a “poor man’s” cluster between two virtual machines and share information quickly between them by using the multi-writer disk. I have used these for a number of different reasons. There are several different use cases for using a multi-writer disk on a virtual machine.
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