VMs not in Write-Back mode because of Multi-Writer VMDK
In rare cases, one or more VMDKs on a VM could have its ‘sharing’ field (drop-down field in ‘Edit Settings’ at the VM level) set to multi-writer, in which case it is not possible to move that VM to Write-Back caching mode. Such VMs can only be set to ‘Write-Through’ caching mode. This means that for such VMs, VirtuCache will only cache reads, but not writes.
Multi Writer VMDKs are typically used in Active-Passive / Active-Active clustering of Databases, where Database instances are running in multiple VMs that might be sharing the same VMDK(s). As the name suggests a multi-writer VMDK allows multiple VMs to write to the same VMDK(s).
Since in Write-Back caching mode, VirtuCache caches writes to local cache media without synchronously writing to the backend storage where the VMDK resides, these VMs that share a VMDK using the multi writer mode option will not see the most current state of the VMDK and hence there will be data consistency issues if a VM with a multi-writer VMDK is set to Write-Back caching mode in VirtuCache. As a result, VirtuCache doesn’t allow VMs that have one or more multi writer VMDKs to be set to Write-Back caching mode. However these VMs can be set to Write-Through caching mode, which will accelerate reads but not writes.