Allocating cache
You need to allocate cache to a virtual machine before you can enable the VM for caching. Cache allocation reserves a portion of the flash device on the ESXi host for use as cache by applications running on the VM.
Before you begin 
Flash Accel requires additional virtual machine memory to support the amount of cache allocated to the VM. The rule of thumb is to configure 0.0035 GB of additional memory for every GB of cache space allocated. So if you are planning to allocate 500 GB to the VM, you should configure an additional 1.75 GB of memory.
Flash Accel uses a physical Raw Device Mappings (pRDM) device to represent the cache to the virtual machine. The RDMP file for the device cannot be stored in an NFS datastore. If the files for the VM are stored in an NFS datastore, its host must have access to a VMFS datastore for the RDMP file.
Note: The maximum cache allocation for a VM using a VMFS-3 datastore is 2 TB minus 256 MB. If you need to allocate more cache to a VM, use a VMFS-5 datastore.
If you have enabled vMotion migration, the amount of cache space you can allocate to the VM is limited not only by the cache space available on its host, but also by the cache space available on its potential migration targets. In other words, if the VM resides on host X, with 100 GB of unallocated cache, and host Y and host Z are both potential migration targets, with 75 GB and 50 GB of unallocated cache, respectively, the maximum cache space you can allocate to the VM is 50 GB. If you are planning to migrate VMs, use a shared VMFS datastore accessible to each of the potential migration target hosts.
If the VM host and migration targets are members of a cluster with VMware DRS/HA enabled, you should add VM-Host affinity rules to the DRS cluster configuration. Adding affinity rules ensures that the VM is provisioned only on Flash Accel-enabled hosts.
Steps 
1.
From the VM Inventory page of the FAMC, click the wrench icon for the virtual machine on which you want to allocate cache and select Cache Space > Allocate.
The Cache Space dialog box opens.
2.
Leave the check boxes for Enable caching for the VM and its disks and Enable cache metrics collection for the VM and its disks selected if you want to enable the VM for caching and cache metrics collection.
3.
Note: To enter the cache size manually, click the Enter Manually check box, then enter the cache size in the Enter Cache Space Size field.
4.
Click Allocate.
Result 
The Cache Size field for the virtual machine is set to the specified value. The VM is listed on the FAMC home page.
Parent Topic: Setting the cache size for virtual machines