Can you combine vSphere Host Cache and vFlash on a single SSD?

One of the new features in vSphere 5.5 is the vSphere vFlash that enables you to use a SSD/Flash device as a read cache for your storage. Duncan Epping has a series of posts on vSphere Flash Cache that is well worth a read.

vSphere vFlash caches your read IOs, but at the same time you can use it as a swap device if you run into memory contention issues. The vSphere vFlash Host Cache is similar to the older Host Cache feature, but if you are upgrading from an older version of ESXi there is a couple of things that needs to be done to be able to use this feature. Screen Shot 2013-09-30 at 02.19.57

If you had the “old” Host Cache enabled before upgrading to v5.5, you have to delete the dedicated Host Cache datastore and re-create a new vSphere vFlash resource to be able to use both vFlash Host Cache and vSphere Flash Read Cache on the same SSD/Flash device.

Also note that vFlash Read Cache is only available for VMs that run in ESXi 5.5 Compatibility Mode aka Virtual Hardware Version 10, and is enabled pr. VMDK in the VMs settings.

Screen Shot 2013-09-30 at 02.19.57Now you can utilize vFlash to both accelerate your read IOs, and speed up your host if you run into swapping issues. Good deal!


  1. If the storage has been formatting w/ VMFS and has VMs running on it, are ‘we’ NOT able to “create a new vSphere vFlash resource”? When I go to ‘add vrutal flash resource capacity’ I see no free SSD and it says its going to format it. So if I had it I assume its no longer VMFS, correct?

    The older Host Cache didn’t seem to mind if VMFS was seutp and VMs were also running on it..

  2. Old host cache is a bunch of files on a vmfs partition that may or may not be SSD. The disk must be provisioned before use.
    vFlash read and host (5.5 hw 10) cache requires an unprovisioned SSD lun. The disk must NOT be provisioned and should have enable_local and enable_ssd. It puts the ssd filesystem on it.
    If you like read caching and host cashing for vmhw before 10, then use vflash just for the read cache and the old host cache for that. This requires 2 SSD drives because they require different file systems VMFS and VFFS.

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